This one is due to commenter P.N., who raised it on another post, and the glaring issue has been discussed there. Still, for the record it should be WitCHed, and we’ve also decided to expand the WitCHiness slightly (and could have expanded it further).
The obvious problem here, of course, is that the answer for Part (b), in both the examination report and VCAA’s Mathematica solutions, is flat out wrong: the function fk will also fail to have a stationary point if k = -2 or k = 0. Nearly as bad, and plenty bad, the method in VCAA’s Mathematica solutions to Part (c) is fundamentally incomplete: for a (twice-differentiable) function f to have an inflection point at some a, it is necessary but not sufficient to have f’’(a) = 0.
That’s all pretty awful, but we believe there is worse here. The question is, how did the VCAA get it wrong? Errors can always occur, but why specifically did the error in Part (b) occur, and why, for a year and counting, wasn’t it caught? Why was a half-method suggested for Part (c), and why was this half-method presumably considered reasonable strategy for the exam? Partly, the explanation can go down to this being a question from NHT, about which, as far as we can tell, no one really gives a stuff. This VCAA screw-up, however, points to a deeper, systemic and much more important issue.
The first thing to note is that Mathematica got it wrong: the Solve function did not return the solution to the equation fk‘ = 0. What does that imply for using Mathematica and other CAS software? It implies the user should be aware that the machine is not necessarily doing what the user might reasonably think it is doing. Which is a very, very stupid property of a black box: if Solve doesn’t mean “solve”, then what the hell does it mean? Now, as it happens, Mathematica’s/VCAA’s screw-up could have been avoided by using the function Reduce instead of Solve.* That would have saved VCAA’s solutions from being wrong, but not from being garbage.
Ask yourself, what is missing from VCAA’s solutions? Yes, yes, correct answers, but what else? This is it: there are no functions. There are no equations. There is nothing, nothing at all but an unreliable black box. Here we have a question about the derivatives of a function, but nowhere are those derivatives computed, displayed or contemplated in even the smallest sense.
For the NHT problem above, the massive elephant not in the room is an expression for the derivative function:
What do you see? Yep, if your algebraic sense hasn’t been totally destroyed by CAS, you see immediately that the values k = 0 and k = -2 are special, and that special behaviour is likely to occur. You’re aware of the function, alert to its properties, and you’re led back to the simplification of fk for these special values. Then, either way or both, you are much, much less likely to screw up in the way the VCAA did.
And that always happens. A mathematician always gets a sense of solutions not just from the solution values, but also from the structure of the equations being solved. And all of this is invisible, is impossible, all of it is obliterated by VCAA’s nuclear weapon approach.
And that is insane. To expect, to effectively demand that students “solve” equations without ever seeing those equations, without an iota of concern for what the equations look like, what the equations might tell us, is mathematical and pedagogical insanity.
*) Thanks to our ex-student and friend and colleague Sai for explaining some of Mathematica’s subtleties. Readers will be learning more about Sai in the very near future.
This post is motivated by a sub-discussion on another post. Mathematical induction is officially in the VCE curriculum (in Specialist 12), but is not there in a properly meaningful sense. So, if people want to suggest what should be done or, the real purpose of this blog, simply wish to howl at the moon, here’s a place to do it.
We’ve written about MAV’s censorship previously. It seems, unfortunately, that we may have another such incident to write about in the near future. We’ll see.
There is also a third incident that we’ve long planned to write about, but have never gotten around to. It is rather involved, and we won’t give the full story here, but one specific aspect is perhaps worth telling now.
In 2016, we accepted an invitation from the MAV to give a keynote address at their Annual Conference. We chose as our keynote title Same Sermon, New Jokes. We also submitted a “bio pic” – the graphic above – and an abstract. The abstract indicated our contempt for twenty or so organisations and facets of Australian mathematics education.
A couple months later, the Conference organisers emailed to indicate their objection to our abstract. One can argue the merits of and the propriety of this objection, and we will write generally on this at a later date, but one aspect of the objection was particularly notable. The email included the following:
“While we welcome all points of view, we do need to be respectful of the organisations we work with, and with whom we need to maintain good relations … We would like you to re visit the text … without the criticism of formal organisations.”
We pushed back against the criticism, and ended our reply with what we intended as a rhetorical question:
“You wrote that you (plural) welcome all points of view, which I was very reassured to read. Given that, which formal organisations do you consider to be above criticism?”
The email reply from the organisers included a response:
“In regards to the formal organisations with which the MAV has relations, you have stated some of them, e.g. ACARA, VCAA.”
No one at the MAV, including the then President, indicated to us any problem with this request or its clarification.
“All students were awarded [the] mark for this question.”
Yep, the question is plain stuffed. We think, however, there is more here than the simple wrongness, which is why we’ve made it a WitCH rather than a PoSWW. Happy hunting.
UPDATE (11/05) Steve C’s comment below has inspired an addition:
The third greatest issue with the exam question is that it is wrong: none of the available answers is correct. The second greatest issue is that the wrongness is obvious: if lies in a sector then the natural guess is that will lie in one of three equally spaced sectors of a third the width, so God knows why the alarm bells weren’t ringing. The greatest issue is that VCAA didn’t have the guts or the basic integrity to fess up: not a single word of responsibility or remorse. Assholes.
Those are the elephants stomping through the room but, as commenters as have noted, there is plenty more awfulness in this question:
“Letting” is sloppy, confusing and pointless;
The term “quadrant” is undefined;
The use of “principal” is unnecessary;
“argument” is better thought as the measure of an angle not the angle itself;
Given is a single complex number, “the complete set of values for Arg()” will consist of a single number.
SACs may not be the greatest problem with VCE mathematics, but they’re right up there. SACs are torture for teachers and torture for students. They teach nothing. As assessment, they are unnecessary, unreliable and phenomenally inefficient. They are a license for VCAA’s unaswerable auditors applying Kafkaesque rules to act either as favour-givers or as little Hitlers, as the mood takes them. These problems are currently amplified to eleven by VCAA’s “We’ll give you some kind of guidance in, oh, a little while” plan for the plague year.
For all of the awfulness of the above, that’s not the worst of it. The worst is that the majority of SACs are monumentally stupid. Literally. A SAC has the imposing presence of a monument, its towering stupidity casting a shadow over everything.
How are SACs so bad? Many contain errors, often subtle although too often not, but, as irritating as that is, that is not the main problem. The main problem is that they are mathematical nonsense. Typically they will present the student with a ridiculous model of a contrived problem, which is then all redone in greater, brain-bludgeoning generality by throwing in a needless parameter in a randomly chosen location. All of this is undertaken, of course, in the nihilistic world of CAS. Finally, somewhere near the end, the poor beleaguered student, who by this stage just wants to escape with their life, will be required to “comment on the model”, to which the usual response is “It’s really nice, please let me go” and to which the only reasonable response is “It’s fucking insane”.
How do we know SACs are this bad? Because we see them. We see the commercial SACs, and the sample SACs, and the past SACs, and the current SACs. Are they all as bad as we suggest? No, of course not. Specialist SACs are typically nowhere near as bad as Methods SACs, and even many Methods SACs will fall short of truly idiotic torture, rising only to the level of being dumb and painful. Then there are the rare few SACs we see that are good, resulting in an exchange:
“This actually makes sense. Who’s your teacher?”
“Oh, it’s Mr. ….”
So, yes, the quality and worth of SACs varies widely, but the average is squarely in the neighbourhood of monumental, tortuous stupidity. Which bring us to the “why”. Why are SACs in general so awful? There are two reasons.
The first and fundamental reason is the VCAA and their view of what they imagine is a curriculum. VCE mathematics subjects are so shallow and so lacking in a foundation of solid reason, that almost any attempt at depth and substance in a SAC is destined to be farce. The VCAA has replaced foundation and depth with CAS, which reaches peak awfulness in SACs. The VCAA promotes the fantasy that CAS magically transforms students into mathematical explorers, clever little Lewises and Clarks skilfully navigating the conceptual wilderness. The reality, of course, is much less Lewis and Clark than it is Burke and Wills. To top it off, SACs must follow guidelines that Terry Gilliam would be proud of, giving us Burke and Wills’ Bogus Brazilian Journey. Or, just Eraserhead. Something like that.
The second reason is the teachers. Sort of. Even if the subjects were coherent, even if they were unpoisoned by CAS and were unconstrained by vague and ridiculous conditions, even then writing a good SAC would be a very difficult and massively time-consuming task. Most teachers just don’t have the mathematical background, or the literary skill, to write a coherent, correct and mathematically rich SAC; many cannot even recognise one. And, that’s writing a good SAC for this imaginary good subject; writing a good SAC for these fundamentally flawed subjects with their ridiculous constraints is close to impossible, even for a strong teacher. And which teachers, particularly weaker teachers, have the time to compose such a good SAC? Why bother trying? And so, with the greatest common sense, most teachers do not. Most teachers stick to the audit-proof and meaningless formulaic SAC bullshit that the VCAA expects and effectively demands.
The VCAA’s SAC system is a crime against mathematical humanity.
We received the following from a student acquaintance (who hadn’t read this post):
Hi Marty, given the upcoming math SACs approaching soon, the pressure is on to practice and practice. Attached below is last year’s Methods SAC1 (Unit 3/4) for [the student’s school]. I remember many talented friends of mine who were stumped, and didn’t do very well on this SAC. Personally, I thought this SAC was horrifying. In contrast to Specialist, (I actually quite enjoy Specialist!), Methods seems to be a huge prick because of frustrating, ambiguous SACs containing questions seemingly cooked from the pits of hell itself. Are these sort of SACs common across the state?
The student is, of course, correct. The SAC, which comes from a highly respected school, is a nightmare in all of the ways canvassed above. From start to end it is idiotic CAS-driven pseudo-modelling, complete with Magritte nonsense and a pointlessly prissy grading scheme. And, yes, the SAC contains an error.
Of course we won’t reveal the school, much less any teachers involved, which means that we are also unable to critique the SAC in detail. But that is one of the insidious aspects of the SAC system; an entirely proper concern for privacy means that SAC nonsense, although endemic, fails to be exposed to the public critique that is so very much needed.
This post concerns a question on the 2019 VCE Specialist Mathematics Exam 2 and, in particular, the solution and commentary for that question available through the Mathematical Association of Victoria. As we document below, a significant part of what MAV has written on this question is confused, self-contradictory and tendentious. Thus, noting the semi-official status of MAV solutions, that these solutions play a significant role in MAV’s Meet the Assessors events, and are quite possibly written by VCE assessors, there are some troubling implications.
Question 3, Section B on Exam 2 is a differential equations problem, with two independent parts. Part (a) is a routine (and pretty nice) question on exponential growth and decay.* Part (b), which is our concern, considers the differential equation
for t ≥ 0, along with the initial condition
The differential equation is separable, and parts (i) and (ii) of the question, worth a total of 3 marks, asks to set up the separation and use this to show the solution of the initial value problem is
Part (iii), worth 2 marks, then asks to show that “the graph of Q as a function of t” has no inflection points.**
Question 3(b) is contrived and bitsy and hand-holding, but not incoherent or wrong. So, pretty good by VCE standards. Unfortunately, the MAV solution and commentary to this problem is deeply problematic.
The first MAV misstep, in (i), is to invert the derivative, giving
prior to separating variables. This is a very weird extra step to include since, not only is the step not required here, it is never required or helpful in solving separable equations. Its appearance here suggests a weak understanding of this standard technique. Worse is to come in (iii). Before considering MAV’s solution, however, it is perhaps worth indicating an approach to (iii) that may be unfamiliar to many teachers and students and, possibly, the assessors.
If we are interested in the inflection points of Q,*** then we are interested in the second derivative of Q. The thing to note is we can naturally obtain an expression for Q” directly from the differential equation: we differentiate the equation using the chain rule, giving
Now, the exponential is never zero, and so if we can show Q’ < 1 then we’d have Q” > 0, ruling out inflection points. Such conclusions can sometimes be read off easily from the differential equation, but it does not seem to be the case here. However, an easy differentiation of the expression for Q derived in part (ii) gives
The numerator is clearly smaller than the denominator, proving that Q’ < 1, and we’re done.
For a similar but distinct proof, one can use the differential equation to replace the Q’ in the expression for Q”, giving
Again we want to show the second factor is positive, which amounts to showing Q > t. But that is easy to see from the expression for Q above (because the stuff in the log is greater than ), and again we can conclude that Q has no inflection points.
One might reasonably consider the details in the above proofs to be overly subtle for many or most VCE students. Nonetheless the approaches are natural, are typically more efficient (and are CAS-free), and any comprehensive solutions to the problem should at least mention the possibility.
The MAV solutions make no mention of any such approach, simply making a CAS-driven beeline for Q” as an explicit function of t. Here are the contents of the MAV solution:
Part 1: A restatement of the equation for Q from part (ii), which is then followed by
Part 2: A screenshot of the CAS input-output used to obtain the conclusion of Part 1.
Part 3: The statement
Solving .˙. gives no solution
Part 4: A screenshot of the CAS input-output used to obtain the conclusion of Part 3.
Part 5: The half-sentence
We can see that for all t,
Part 6: A labelled screenshot of a CAS-produced graph of Q”.
Part 7: The second half of the sentence,
so Q(t) has no points of inflection
This is a mess. The ordering of the information is poor and unexplained, making the unpunctuated sentences and part-sentences extremely difficult to read. Part 3 is so clumsy it’s funny. Much more important, the MAV “solution” makes little or no mathematical sense and is utterly useless as a guide to what the VCE might consider acceptable on an exam. True, the MAV solution is followed by a commentary specifically on the acceptability question. As we shall see, however, this commentary makes things worse. But before considering that commentary, let’s itemise the obvious questions raised by the MAV solution:
Is using CAS to calculate a second derivative on a “show that” exam question acceptable for VCE purposes?
Can a stated use of CAS to “show” there are no solutions to Q” = 0 suffice for VCE purposes? If not, what is the purpose of Parts 3 and 4 of the MAV solutions?
Does copying a CAS-produced graph of Q” suffice to “show” that Q” > 0 for VCE purposes?
If the answers to the above three questions differ, why do they differ?
Yes, of course these questions are primarily for the VCAA, but first things first.
The MAV solution is followed by what is intended to be a clarifying comment:
Note that any reference to CAS producing ‘no solution’ to the second derivative equalling zero would NOT qualify for a mark in this ‘show that’ question. This is not sufficient. A sketch would also be required as would stating for all t.
These definitive-sounding statements are confusing and interesting, not least for their simple existence. Do these statements purport to be bankable pronouncements of VCAA assessors? If not, what is their status? In any case, given that pretty much every exam question demands that students and teachers read inscrutable VCAA tea leaves, why is it solely the solution to question 3(b) that is followed by such statements?
The MAV commentary at least makes clear their answer to our second question above: quoting CAS is not sufficient to “show” that Q” = 0 has no solutions. Unfortunately, the commentary raises more questions than it answers:
Parts 3 and 4 are “not sufficient”, but are they worth anything? If so, what are they worth and, in particular, what is the import of the word “also”? If not, then why not simply declare the parts irrelevant, in which case why include those parts in the solutions at all?
If, as claimed, it is “required” to state (which is indeed the key point of this approach and should be required), then why does the MAV solution not contain any such statement, nor even the factorisation that would naturally precede this statement?
Why is a solution “required” to include a sketch of Q”? If, in particular, a statement such as is “required”, or in any case is included, why would the latter not in and of itself suffice?
We wouldn’t begin to suggest answers to these questions, or our four earlier questions, and they are also not the main point here. The main point is that under no circumstances should such shoddy material be the basis of VCAA assessor presentations. If the material was also written by VCAA assessors, all the worse.
Of course the underlying problem is not the quality or accuracy of solutions but, rather, the fundamental idiocy of incorporating CAS into proof questions. And for that the central villain is not the MAV but the VCAA, which has permitted their glorification of technology to completely destroy the appreciation of and the teaching of proof and reason.
The MAV is not primarily responsible for this nonsense. The MAV is, however, responsible for publishing it, promoting it and profiting from it, none of which should be considered acceptable. The MAV needs to put serious thought into its unhealthily close relationship with the VCAA.
*) We might ask, however, who refers to “The growth and decay” of an exponential function?
**) One might simply have referred to Q, but VCAA loves them their words.
***) Or, if preferred, the points of inflection of the graph of Q as a function of t.
The Examination Report is out and is basically ok; none of the nonsense and non sequiturs of the MAV solutions are included. The solution to (b)(iii) correctly focuses upon the factoring of Q”, although it needlessly worries about the sign of the denominator. There is no mention of the more natural approach to obtaining and analysing Q” but, given the question is treated by the VCAA and pretty much everyone as just another mindless exercise in pushing buttons, this is no surprise.
What a mess. Thanks to Damo for pointing out the problem, and thanks to the commenters for figuring out the nonsense.
In general form, the (intended) scenario of the exam question is
Thevector resolute of in the direction of is ,
which can be pictured as follows: For the exam question, we have , and .
Of course, given and it is standard to find . After a bit of trig and unit vectors, we have (in must useful form)
The exam question, however, is different: the question is, given and , how to find .
The problem with that is, unless the vectors and are appropriately related, the scenario simply cannot occur, meaning cannot exist. Most obviously, the length of must be no greater than the length of . This requirement is clear from the triangle pictured, and can also be proved algebraically (with the dot product formula or the Cauchy-Schwarz inequality).
This implies, of course, that the exam question is ridiculous: for the vectors in the exam we have , and that’s the end of that. In fact, the situation is more delicate; given the pictured vectors form a right-angled triangle, we require that be perpendicular to . Which implies, once again, that the exam question is ridiculous.
Next, suppose we lucked out and began with perpendicular to . (Of course it is very easy to check whether we’ve lucked out.) How, then, do we find ? The answer is, as is made clear by the picture, “Well, duh”. The possible vectors are simply the (non-zero) scalar multiples of , and we’re done. Which shows that the mess in the intended solution, Answer A, is ridiculous.
There is a final question, however: the exam question is clearly ridiculous, but is the question also stuffed? The equations in answer A come from the equation for above and working backwards. And, these equations correctly return no solutions. Moreover, if the relationship between and had been such that there were solutions, then the A equations would have found them. So, completely ridiculous but still ok?
The question is framed from start to end around definite, existing objects: we have THE vector resolute, resulting in THE values of m, n and p. If the VCAA had worded the question to find possible values, on the basis of a possible direction for the resolution, then, at least technically, the question would be consistent, with A a valid answer. Still an utterly ridiculous question, but consistent. But the VCAA didn’t do that and so the question isn’t that. The question is stuffed.
Further Update (26/06/20)
As commenters have noted, the Examination Report has finally appeared. And, as predicted, answer A was deemed correct, with the Report noting
Option A gives the set of equations that can be used to obtain the values of m, n and p. Explicit solution would result in a null set as it is not possible for a result of a vector to be of greater magnitude than the vector itself.
Well, it’s something. Presumably “result of a vector” was intended to be “resolute of a vector”, and the set framing is weirdly New Mathy. But, it’s something. Seriously. As John Friend notes, it is at least a small step along the way to indicating the question is not all hunky-dory.
That step, however, is way too small. We’ll close with two comments, reiterating the points made above.
1. The question is wrong
Read the question again, and read the first sentence of the Report’s comment. The question and report justification are fundamentally stuffed by the definite articles, by the language of existence. All answers should have been marked correct.
2. The question is worse than wrong
Even if the vectors and had been chosen appropriately, the question is utterly devoid of mathematical sense. It suggests a long and difficult method to solve a problem that, if indeed is solvable, is trivial.
It seems that what amounts to VCE exam marking schemes may be available for purchase through the Mathematical Association of Victoria. This seems very strange, and we’re not really sure what is going on, but we shall give our current sense of it. (It should be noted at the outset that we are no fan of the MAV in its current form, nor of the VCAA in any form: though we are trying hard here to be straightly factual, our distaste for these organisations should be kept in mind.)
Each year, the MAV sells VCE exam solutions for the previous year’s exams. It is our understanding that it is now the MAV’s strong preference that these solutions will be written by VCAA assessors. Further, the MAV is now advertising that these solutions are “including marking allocations“. We assume that the writers are paid by the MAV for this work, and we assume that the MAV are profiting from the selling of the product, which is not cheap. Moreover, the MAV also hosts Meet the Assessors events which, again, are not cheap and are less cheap for non-members of the MAV. Again, it is reasonable to assume that the assessors and/or the MAV profit from these events.
We do not understand any of this. One would think that simple equity requires that any official information regarding VCE exams and solutions should be freely available. What we understand to be so available are very brief solutions as part of VCAA’s examiners’ reports, and that’s it. In particular, it is our understanding that VCAA marking schemes have been closely guarded secrets. If the VCAA is loosening up on that, then that’s great. If, however, VCAA assessors and/or the MAV are profiting from such otherwise unavailable information, we do not understand why anyone should regard that as acceptable. If, on the other hand, the MAV and/or the assessors are not so profiting, we do not understand the product and the access that the MAV is offering for sale.
We have written previously of the worrying relationship between the VCAA and the MAV, and there is plenty more to write. On more than one occasion the MAV has censored valid criticism of the VCAA, conduct which makes it difficult to view the MAV as a strong or objective or independent voice for Victorian maths teachers. The current, seemingly very cosy relationship over exam solutions, would only appear to make matters worse. When the VCAA stuffs up an exam question, as they do on a depressingly regular basis, why should anyone trust the MAV solutions to provide an honest summary or evaluation of that stuff up?
Again, we are not sure what is happening here. We shall do our best to find out, and commenters, who may have a better sense of MAV and VCAA workings, may comment (carefully) below.
As John Friend has indicated in his comment, the “marking allocations” appears to be nothing but the trivial annotation of solutions with the allotted marks, not a break-down of what is required to achieve those marks. So, simply a matter of the MAV over-puffing their product. As for the appropriateness of the MAV being able to charge to “meet” VCAA assessors, and for solutions produced by assessors, those issues remain open.
We’ve also had a chance to look at the MAV 2019 Specialist solutions (not courtesy of JF, for those who like to guess such things.) More pertinent would be the Methods solutions (because of this, this, this and, especially, this.) Still, the Specialist solutions were interesting to read (quickly), and some comments are in order. In general, we thought the solutions were pretty good: well laid out with usually, though not always, the seemingly best approach indicated. There were a few important theoretical errors (see below), although not errors that affected the specific solutions. The main general and practical shortcoming is the lack of diagrams for certain questions, which would have made those solutions significantly clearer and, for the same reason, should be encouraged as standard practice.
For the benefit of those with access to the Specialist solutions (and possibly minor benefit to others), the following are brief comments on the solutions to particular questions (with section B of Exam 2 still to come); feel free to ask for elaboration in the comments. The exams are here and here.
Q5. There is a Magritte element to the solution and, presumably, the question.
Q6. The stated definition of linear dependence is simply wrong. The problem is much more easily done using a 3 x 3 determinant.
Q7. Part (a) is poorly set out and employs a generally invalid relationship between Arg and arctan. Parts (c) and (d) are very poorly set out, not relying upon the much clearer geometry.
Q8. A diagram, even if generic, is always helpful for volumes of revolution.
Q9. The solution to part (b) is correct, but there is an incorrect reference to the forces on the mass, rather than the ring. The expression “… the tension T is the same on both sides …” is hopelessly confused.
Q10. The question is stupid, but the solutions are probably as good as one can do.
Exam 2 (Section A)
MCQ5. The answer is clear, and much more easily obtained, from a rough diagram.
MCQ6. The formula Arg(a/b) = Arg(a) – Arg(b) is used, which is not in general true.
MCQ11. A very easy question for which two very long and poorly expressed solutions are given.
MCQ12. An (always) poor choice of formula for the vector resolute leads to a solution that is longer and significantly more prone to error. (UPDATE 14/2: For more on this question, go here.)
MCQ13. A diagram is mandatory, and the cosine rule alternative should be mentioned.
MCQ14. It is easier to first solve for the acceleration, by treating the system as a whole.
MCQ19. A slow, pointless use of CAS to check (not solve) the solution of simultaneous equations.
Q1. In Part (a), the graphs are pointless, or at least a distant second choice; the choice of root is trivial, since y = tan(t) > 0. For part (b), the factorisation should be noted. In part (c), it is preferable to begin with the chain rule in the form , since no inverses are then required. Part (d) is one of those annoyingly vague VCE questions, where it is impossible to know how much computation is required for full marks; the solutions include a couple of simplifications after the definite integral is established, but God knows whether these extra steps are required.
Q2. The solution to Part (c) is very poorly written. The question is (pointlessly) difficult, which means clear signposts are required in the solution; the key point is that the zeroes of the polynomial will be symmetric around (-1,0), the centre of the circle from part (b). The output of the quadratic formula is neccessarily a mess, and may be real or imaginary, but is manipulated in a clumsy manner. In particular, a factor of -1 is needlessly taken out of the root, and the expression “we expect” is used in a manner that makes no sense. The solution to the (appallingly written) Part (d) is ok, though the centre of the circle is clear just from symmetry, and we have no idea what “ve(z)” means.
Q3. There is an aspect to the solution of this question that is so bad, we’ll make it a separate post. (So, hold your fire.)
Q4. Part (a) is much easier than the notation-filled solution makes it appear.
Q5. Part (c)(i) is weird. It is a 1-point question, and so presumably just writing down the intuitive answer, as is done in the solutions, is what was expected and is perhaps reasonable. But the intuitive answer is not that intuitive, and an easy argument from considering the system as a whole (see MCQ14) seems (mathematically) preferable. For Part (c)(ii), it is more straight-forward to consider the system as a whole, making the tension redundant (see MCQ14). The first (and less preferable) solution to Part (d) is very confusing, because the two stages of computation required are not clearly separated.
Q6. It’s statistical inference: we just can’t get ourselves to care.
The Specialist Maths examination reports are finally, finally out (here and here), so it seems worth revisiting the MAV “Assessor” solutions. In summary, the clumsiness of and errors in the MAV solutions as indicated above (and see also here and here) do not appear in the reports; in the main this is because the reports are pretty much silent on any aspect involving some subtlety. Sigh.
Some specific comments:
Q5 Yes, Magritte-ish. Justifying that the critical points are extrema was not expected, meaning conscientious students wasted their time.
Q6 The error in the MAV solutions is ducked in the report.
Q7 The error in the MAV solutions is ducked in the report.
EXAM 2 (Section A)
MCQ6The error in the MAV solutions is ducked in the report.
MCQ11 The report is silent.
MCQ12 A huge screw-up of a question, to which the report hemidemisemi confesses: see here.
MCQ14 The report suggests the better method for solving this problem.
EXAM 2 (Section B)
Q2 Jesus. This question was intrinsically confusing and very badly worded, with the students inevitably doing poorly. So, why the hell is the examination report almost completely silent? The MAV solutions were a mess, but the absence of comment in the report is disgraceful.
Q3 The solution in the report is ok, although more could have been written. But, it’s not the garbled nonsense of the MAV solution, as detailed here.
We’re not actively looking for WitCHes right now, since we have a huge backlog to update. This one, however, came up in another context and, after chatting about it with commenter Red Five, there seemed no choice.
The following 1-mark multiple choice question appeared in 2019 Exam 2 (CAS) of VCE’s Mathematical Methods.
The problem was to determine Pr(X > 0), the possible answers being
A. 2/3 B. 3/4 C. 4/5 D. 7/9 E. 5/6
Who writes this crap? Who writes such a problem, who proofreads such a problem, and then says “Yep, that’ll work”? Because it didn’t work, and it was never going to. The examination report indicates that 27% of students gave the correct answer, a tick or two above random guessing.
We’ll outline a solution below, but first to the crap. The main awfulness is the double-function nonsense, defining the probability distribution in terms of pretty make the same function . What’s the point of that? Well, of course is defined on all of and is only defined on . And, what’s the point of defining on all of ? There’s absolutely none. It’s completely gratuitous and, here, completely ridiculous. It is all the worse, and all the more ridiculous, since the function isn’t properly defined or labelled piecewise linear, or anything; it’s just Magritte crap.
To add to the Magritte crap, commenter Oliver Oliver has pointed out the hilarious Dali crap, that the Magritte graph is impossible even on its own terms. Beginning in the first quadrant, the point is not quite symmetrically placed to make a angle. And, yeah, the axes can be scaled differently, but why would one do it here? But now for the Dali: consider the second quadrant and ask yourself, how are the axes scaled there? Taking a hit of acid may assist in answering that one.
Now, finally to the problem. As we indicated, the problem itself is fine, it’s just weird and tricky and hellishly long. And worth 1 mark.
As commenters have pointed out, the problem doesn’t have a whole lot to do with probability. That’s just a scenario to give rise to the two equations,
The problem is then to evaluate
The problem is tricky, not least because it feels as if there may be an easy way to avoid the full-blown simultaneous equations. This does not appear to be the case, however. Of course, the VCAA just expects the lobotomised students to push the damn buttons which, one must admit, saves the students from being tricked.
Anyway, for the non-lobotomised among us, the simplest approach seems to be that indicated below, by commenter amca01. First multiply equation (1) by 2 and rearrange, to give
Then, plugging in (2), we have
That then plugs into **), giving the answer 7/9.
Very nice. And a whole 90 seconds to complete, not counting the time lost making sense of all the crap.