MAV’s Dangerous Inflection

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February 24, 2020

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15 Comments on MAV’s Dangerous Inflection

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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

    \[\boldsymbol{\color{blue}\frac{{\rm d}Q}{{\rm d}t\ } = e^{t-Q}}\,,\]

for t ≥ 0, along with the initial condition

    \[\boldsymbol{\color{blue}Q(0) =1}\,.\]

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

    \[\boldsymbol{\color{blue}Q =\log_e\hspace{-1pt} \left(e^t + e -1\right)}\,.\]

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

    \[\boldsymbol{\color{red}\frac{{\rm d}t\ }{{\rm d}Q } = e^{Q-t}}\,,\]

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

    \[\boldsymbol{\color{magenta}Q'' = e^{t-Q}\left(1 - Q'\right)}\,.\]

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

    \[\boldsymbol{\color{magenta}Q' =\frac{e^t}{e^t + e -1}}\,.\]

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

    \[\boldsymbol{\color{magenta}Q'' = e^{t-Q}\left(1 - e^{t-Q}\right)}\,.\]

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 \boldsymbol{e^t}), 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 

.˙.  \boldsymbol{ \color{red}\ \frac{{\rm d}^2Q }{{\rm d}t^2\ } = \frac{e^{t+1} -e^t}{\left(e^t + e -1\right)^2} } 

Part 2: A screenshot of the CAS input-output used to obtain the conclusion of Part 1.

Part 3: The statement   

Solving  .˙.  \boldsymbol{\color{red} \ \frac{{\rm d}^2Q }{{\rm d}t^2\ } = 0} 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 \boldsymbol{\color{red}\frac{{\rm d}^2Q }{{\rm d}t^2\ } > 0} 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 \boldsymbol{\color{red}e^t (e - 1) \neq 0} 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 \boldsymbol{e^t(e-1)\neq 0} (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 \boldsymbol{e^t(e-1)\neq 0} 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.

Update (26/06/20)

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.

Related posts:

  1. The Troubling Cosiness of the VCAA and the MAV
  2. MAV’s Mathematica Games
  3. The MAV and a Matter of Opinion
  4. The Madness of Crowd Models
  5. MAV’s Trials and Tribulations
  6. Hard SEL: The Specialist Error List

15 Replies to “MAV’s Dangerous Inflection”

  1. The most disheartening aspect of all this is that I don’t think the MAV will ever acknowledge that its inbred solutions are substandard (NB: This is my opinion, based on the evidence).

    ‘Written by VCAA Assessors’ is a marketing tool intended to imply quality and reliability, regardless of the reality. Unfortunately, many teachers buying this (and the Meet the Assessors) products are generally not interested in critically evaluating these products, being happy instead with near enough is good enough. (If this was not the case, then surely Meet the Assessor would not be as popular as it apparently is, and there would be a much stronger demand by teachers for better quality).

    It’s little wonder that many teachers (including myself) much prefer the FREE products readily available on-line. They may have similar issues but at least you’re not paying hundred of dollars, and the clear implication of semi-official status is not trumpeted. The free solutions are also willing to acknowledge mistakes and publish errata ….

    Reply

    1. Thanks, JF. Yes, it’s difficult to imagine the MAV or VCAA ever publicly admitting they stuffed up. Whether this behaviour stems from arrogance or cluelessness, I’m not sure. (Yeah, yeah, the taco commercial …)

      I’m not sure the MAV or the VCAA will get away with this so easily, however. These solutions are written evidence of the basis of meetings with the assessors. If there’s clear error or blatant nonsense, it is difficult to deny or to ignore. The question above is particularly difficult to ignore because, not only is the solution a complete mess, there is explicit and totally confused advice on what is supposedly a VCE-acceptable solution.

      Reply

  2. I cannot wait to see the VCAA “report” on this exam, at the very least to see what students, on the whole, made of this mess.

    But specifically to this question, it will be really interesting to see VCAA’s “answer” since we have all heard at one point or another that “show that and verify are not the same thing”

    Reply

    1. Jesus. I’m not going to ask for the difference between “show that” and “verify”. What a tiny, idiotic world.

      But yes, the reports on all the exams will be interesting. There were plenty of dodgy questions on all four MM/SM exams from last year, and the MAV/assessor solutions to both Specialist exams have clear flaws. (I haven’t see the MM solutions, but I’m willing to bet what they’re like.)

      Reply

      1. The MAV MM Exam 2 ‘solutions’ could not possibly be worse than the 2016 ‘solutions’, that make no mention of the erroneous probability density function but just go ahead and blithely calculate medians and means.

        Provable fact 1: Those 2016 solutions were written by a member of a VCAA exam setting panel.
        Provable fact 2: The same person is quoted as saying: “…. comments about VCAA have to be removed [from MAV solutions]. We are supposed to be writing solutions for MAV, not referring to VCAA.”

        Solutions to VCAA exams (and presentations purporting to discuss VCAA exams) will NEVER be accurate when written and/or influenced by VCAA stooges. Irrespective of advertising to the contrary. There is an insurmountable conflict of interest.

        It’s long overdue for mathematics teachers to demand BETTER products for their time and money and not betrayed by rampant conflicts of interest and misleading advertising.

        Reply

        1. I think there is more here than just conflict of interest. The VCAA is in and of itself not in conflict: it is simply incompetent or dishonest, or both, depending upon the particular issue. Similarly, the MAV has done plenty that is inept, simply through being inept. Of course the type of MAV-VCAA conflict of interest you mention, and which has reared its head on more than one occasion, is shameful and makes matters much worse.

          There was also blatant dishonesty before 2016, at least from the VCAA. The 2014 MM2 Exam Report had, and has, a blatant falsehood, and it is almost certainly a falsehood of which the VCAA was and is aware. That is, it’s a blatant fucking lie. I’ll bet London to a brick that deliberate dishonesty has been long-standing practice.

          However, I still think the issue of current MAV solutions is different, and cannot be swept away in the same manner. This time, I think the MAV has bitten off more conflict than they can chew.

          Reply

  3. Marty, for those of us with the memory of a goldfish, can you remind me what the falsehood is in the 2014 MM Exam 2 Report.

    And does anyone know whether this falsehood was/is perpetuated elsewhere (commercial exam solutions, presentations etc), and where it’s been called out (apart from here)?

    Reply

    1. JF, It was MCQ14. See here. Note that the error in the question was not as own-goalish or as significant as other VCAA exam errors and idiocies, although it certainly was a bad error. The main point is that the dishonesty in (and, hilariously, insufficiency of) the examination report is as blatant as it gets.

      Reply

  4. Re: Update – “There is no mention [in the VCAA Examination Report] of the more natural approach to obtaining and analysing Q”.

    In fairness to the VCAA, the Examination Report does not pretend to be solutions:

    “This report provides sample answers or an indication of what answers may have included. Unless
    otherwise stated, these are not necessarily intended to be exemplary or complete responses.”

    Nevertheless one could reasonably argue that the approach explained in this post is in the best interest of the mathematical community and thus should have been mentioned. IF the method was actually included on the marking scheme (doubtful, otherwise one would assume it get mentioned in the dreadful MAV solutions).

    But the MAV solutions DO pretend to be exemplary and complete responses:

    “These solutions have been prepared by current VCAA Assessors, and a panel of experienced reviewers to ensure each solution set is free of errors.
    Each set of solutions feature:
    Fully worked solutions for all sections
    Advice on solution processes”

    The only truth amongst this thicket of unconscionable marketing propaganda is that the MAV solutions were indeed prepared by current (quite senior) VCAA Asses (influential self-proclaimed mathematical mavens who should be ashamed of the fact that they wrote the drivel in those solutions).

    For example:

    1) The (snide) remark “to ensure each solution set is free of errors” impugns every previous Non-VCAA-ass who wrote MAV exam solutions and is totally insulting. And is complete bullshit. The real reason is …. hey, wait a moment …. listen to them little birdies …. cheep cheep.

    Since when is being a VCAA ass the requirement for producing correct and complete mathematical solutions to exams?? In many cases I’ve seen quite the opposite (particularly among some of the more influential self-proclaimed mathematical mavens). 2019 NHT Specialist Maths Exam 2 Q2, anyone …. cheep cheep ….

    2) This post clearly exposes the claim “Fully worked solutions for all sections” as inaccurate.

    3) “Advice on solution processes” is a feature ….? Really? Dubious at best. OK, advice on CAS usage (found in most Examination Reports) is such advice, I suppose. But it’s hardly worth the money these ‘solutions’ cost. The advice I’d be demanding from VCAA ASSES and EXPERIENCED REVIEWERS is stuff like what forms of answer were acceptable, ahem when is a given vector NOT a vector resolute of another vector etc.

    One could argue that the the nepotism shown by MAV in its incestuous relationship with VCAA has actually led to a product inferior to what might have been written by people with NO VCAA affiliation.

    Reply

    1. JF, I’m not at all surprised that the more subtle approaches outlined above appear in neither the MAV solutions nor the examination report. The whole point of VCE mathematics is to think as little as is humanly possible; reflection is considered foolish and unnecessary. To that end, and on its own idiotic terms, the examination report on this question is fine. The MAV solution, however, is a complete mess from any possible perspective. One can only speculate at the reasons, but it smells highly of a rushed job by a person out of their depth.

      Reply

      1. Marty, I think your olfactory sense is spot-on. Cheep cheep ….

        Many of these senior asses built their misleading reputations on the CAS calculator, and so:

        1) Wouldn’t know how to do a lot of this stuff without that crutch.

        2) Have a vested interest in promoting that crutch as much as possible to maintain his/her reputation as a guru (this reputation only has credibility among inexperienced and less mathematically-capable people, but that cohort is always sufficiently large to stoke the asses egos). Look at all the pointless CAS screenshots – what possible useful purpose do they serve? And if MAV insist on inserting such screenshots, then lets include screenshots from:

        TI-Nsipid, Crapio and Mathematicrap.

        (You could include the very minority used Spewlett Packard calculator)

        MAV and VCAA wouldn’t know their asses from a hole in the function ….

        Reply

        1. The philosophy of the VCAA connected ‘senior assessors’ that write this shit is that because you can use a CAS, you should use a CAS. Rather than just because you can use a CAS doesn’t mean that you should.

          The use of CAS-calculator screenshits in the MAV solutions is very stupid because:
          1) It presumes that every student uses that one particular CAS-calculator,
          2) It distracts from the mathematics and, in fact, overtly discourages the mathematics in favour of always using that CAS-calculator,
          3) Many questions on Exam 2 can be done faster without using a CAS.

          The prolific and gratuitous use of CAS-calculator screenshits in these ‘solutions’ is a cheap gimmick used by third-rate writers with limited mathematical knowledge who are desperately trying to justify their writing fee and trying to make their solutions look impressive and authoritative. Unfortunately, this gimmick consistently fools many teachers and most students.

          Instead of filling solutions with vacuous and gratuitous CAS-calculator screenshits, statements such as

          “Use a CAS to solve the equation …”

          followed by a statement of the solutions should be used.
          But due to the limited ability of the people currently writing this crap, their solutions would only be two pages long and would not be seen as worth the price compared to the FREE solutions that can be found on-line. Unfortunately, this is not limited to commercial solutions to Exam 2, it’s also rife in the solutions to commercial trial examination 2.

          I assume that things are just as bad in solutions to 2020 Exam 2, because the same fools are writing this stuff. I’m only guessing because I refuse to buy the crap being pedalled.

          Quality solutions for Exam 2 would have NO CAS-calculator screenshits. They would focus on the maths and the mathematical logic. Anyone looking for decent solutions should refuse to buy solutions full of screenshits. Demand better from the commercial organisations that sell this shit.

          Reply

  5. What I look forward to is seeing the MAV ‘solutions’ (*) for the VCAA 2022 Specialist Maths Exam 2. In particular, the ‘solutions’ to
    Section A Question 4
    Section A Question 19
    Section B Question 6 part (f).

    The MAV ‘solution’ (*) to VCAA Specialist Maths Exam 1 Question 3 part (b) also holds great interest to me.

    * All solutions written loud and proud by VCAA Assessors:
    “These solutions have been prepared by current VCAA Assessors, and a panel of experienced reviewers to ensure each solution set is free of errors.”
    https://www.mav.vic.edu.au/Resources/VCE-Resources

    Reply

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