Year: 2009
Paper: 1
Question Number: 5
Course: LFM Pure
Section: Implicit equations and differentiation
There were significantly more candidates attempting this paper again this year (over 900 in total), and the scores were pleasing: fewer than 5% of candidates failed to get at least 20 marks, and the median mark was 48. The pure questions were the most popular as usual; about two-thirds of candidates attempted each of the pure questions, with the exceptions of question 2 (attempted by about 90%) and question 5 (attempted by about one third). The mechanics questions were only marginally more popular than the probability and statistics questions this year; about one quarter of the candidates attempted each of the mechanics questions, while the statistics questions were attempted by about one fifth of the candidates. A significant number of candidates ignored the advice on the front cover and attempted more than six questions. In general, those candidates who submitted answers to eight or more questions did fairly poorly; very few people who tackled nine or more questions gained more than 60 marks overall (as only the best six questions are taken for the final mark). This suggests that a skill lacking in many students attempting STEP is the ability to pick questions effectively. This is not required for A-levels, so must become an important part of STEP preparation. Another "rubric"-type error was failing to follow the instructions in the question. In particular, when a question says "Hence", the candidate must make (significant) use of the preceding result(s) in their answer if they wish to gain any credit. In some questions (such as question 2), many candidates gained no marks for the final part (which was worth 10 marks) as they simply quoted an answer without using any of their earlier work. There were a number of common errors which appeared across the whole paper. These included a noticeable weakness in algebraic manipulations, sometimes indicating a serious lack of understanding of the mathematics involved. As examples, one candidate tried to use the misremembered identity cos β = sin √(1 − β²), while numerous candidates made deductions of the form "if a² + b² = c², then a + b = c" at some point in their work. Fraction manipulations are also notorious in the school classroom; the effects of this weakness were felt here, too. Another common problem was a lack of direction; writing a whole page of algebraic manipulations with no sense of purpose was unlikely to either reach the requested answer or gain the candidate any marks. It is a good idea when faced with a STEP question to ask oneself, "What is the point of this (part of the) question?" or "Why has this (part of the) question been asked?" Thinking about this can be a helpful guide. One aspect of this is evidenced by pages of formulæ and equations with no explanation. It is very good practice to explain why you are doing the calculation you are, and to write sentences in English to achieve this. It also forces one to focus on the purpose of the calculations, and may help avoid some dead ends. Finally, there is a tendency among some candidates when short of time to write what they would do at this point, rather than using the limited time to actually try doing it. Such comments gain no credit; marks are only awarded for making progress in a question. STEP questions do require a greater facility with mathematics and algebraic manipulation than the A-level examinations, as well as a depth of understanding which goes beyond that expected in a typical sixth-form classroom.
Difficulty Rating: 1484.0
Difficulty Comparisons: 1
Banger Rating: 1484.0
Banger Comparisons: 1
A right circular cone has base radius $r$, height $h$ and
slant length $\ell$. Its volume $V$, and the area $A$ of its curved
surface, are given by
\[ V= \tfrac13 \pi r^2 h \,, \ \ \ \ \ \ \ A = \pi r\ell\,. \]
\begin{questionparts}
\item Given that $A$ is fixed and $r$ is chosen so that $V$ is at its stationary value, show that $A^2 = 3\pi^2r^4$ and that
$\ell =\sqrt3\,r$.
\item Given, instead, that $V$ is fixed and $r$ is chosen so that $A$ is at its stationary value, find $h$ in terms of $r$.
\end{questionparts}\begin{questionparts}
\item Given $A$ is fixed, and $h^2 + r^2 = \ell^2$, we can look at
\begin{align*}
&& V^2 &= \frac19 \pi^2 r^4 h^2 \\
&&&= \frac19\pi^2r^4(\ell^2 - r^2) \\
&&&= \frac19\pi^2 r^4\left (\frac{A^2}{\pi^2r^2} - r^2 \right) \\
&&&= \frac{A^2r^2 - \pi^2r^6}{9}
\end{align*}
Differentiating wrt to $r$ we find that $2rA^2-6\pi^2 r^5 = 0$ or hence $A^2 = 3\pi^2 r^4 \Rightarrow A = \sqrt{3}\pi r^2$.
Therefore $\sqrt{3}\pi r^2 = \pi r \ell \Rightarrow \ell = \sqrt{3}r$.
\item Supposing $V$ is fixed, then
\begin{align*}
&& A^2 &= \pi^2 r^2\ell^2 \\
&&&= \pi^2 r^2 (h^2+r^2) \\
&&&= \pi^2 r^2 \left ( \frac{9V^2}{\pi^2r^4} + r^2 \right) \\
&&&= 9V^2r^{-2} + \pi^2r^4 \\
\end{align*}
Differentiating wrt to $r$ we find $-18V^2r^{-3} + 4\pi^2 r^3 = 0$ so $V^2 = \frac{2\pi^2}{9}r^6$ or $V = \frac{\sqrt{2}\pi}{3}r^3$, from which it follows: $\frac{\sqrt{2}\pi}{3}r^3 = \frac13\pi r^2 h \Rightarrow h = \sqrt{2}r$
\end{questionparts}
This was the least popular of the pure questions, being attempted by barely one-third of the candidates. The marks followed suit; one third of the attempts scored 0 while another third scored 1: these were generally candidates who tried the question and then quickly gave up. Of those who got past the initial stages of the question, the significant majority gained full marks or very close to it. The question essentially required only one idea, and was then a straightforward application of "connected rates of change" or the chain rule. The idea needed was that r, h and ℓ are related via Pythagoras, so that if A is fixed (i.e., it's a constant), then h can be expressed as a function of r. Those students who realised that h is a function of r mostly got the whole question correct; those who regarded h as a constant got nowhere.