OCR C2 2012 January — Question 9 9 marks

Exam BoardOCR
ModuleC2 (Core Mathematics 2)
Year2012
SessionJanuary
Marks9
PaperDownload PDF ↗
Mark schemeDownload PDF ↗
TopicTrigonometric equations in context
TypeSolve tan·sin or tan·trig product
DifficultyStandard +0.3 This is a standard C2 trigonometric equation question with clear scaffolding. Part (i) involves routine graph sketching of transformed trig functions. Part (ii) provides the target form explicitly and uses standard techniques: converting tan to sin/cos, multiplying through, applying Pythagorean identity, then solving a quadratic in sin(x/2). The 'show that' structure removes problem-solving burden, making this slightly easier than average.
Spec1.05f Trigonometric function graphs: symmetries and periodicities1.05j Trigonometric identities: tan=sin/cos and sin^2+cos^2=11.05k Further identities: sec^2=1+tan^2 and cosec^2=1+cot^21.05o Trigonometric equations: solve in given intervals

9
  1. Sketch the graph of \(y = \tan \left( \frac { 1 } { 2 } x \right)\) for \(- 2 \pi \leqslant x \leqslant 2 \pi\) on the axes provided.
    On the same axes, sketch the graph of \(y = 3 \cos \left( \frac { 1 } { 2 } x \right)\) for \(- 2 \pi \leqslant x \leqslant 2 \pi\), indicating the point of intersection with the \(y\)-axis.
  2. Show that the equation \(\tan \left( \frac { 1 } { 2 } x \right) = 3 \cos \left( \frac { 1 } { 2 } x \right)\) can be expressed in the form $$3 \sin ^ { 2 } \left( \frac { 1 } { 2 } x \right) + \sin \left( \frac { 1 } { 2 } x \right) - 3 = 0$$ Hence solve the equation \(\tan \left( \frac { 1 } { 2 } x \right) = 3 \cos \left( \frac { 1 } { 2 } x \right)\) for \(- 2 \pi \leqslant x \leqslant 2 \pi\).

Question 9(i):
AnswerMarks Guidance
Answer/WorkingMarks Guidance
Correct shape for \(y = k\cos\!\left(\frac{1}{2}x\right)\)B1 Must show intention to pass through \((-\pi, 0)\) and \((\pi, 0)\). Should be roughly symmetrical in the \(y\)-axis, but condone slightly different \(y\)-values at \(-2\pi\) and \(2\pi\). Ignore graph outside of given range.
Correct shape for \(y = \tan\!\left(\frac{1}{2}x\right)\)B1 Must show intention to pass through \((-2\pi, 0)\), \((0, 0)\), \((2\pi, 0)\). Asymptotes need not be marked, but there should be no clear overlap of the limbs, nor significant gaps between them. Ignore graph outside of given range.
\((0, 3)\) stated or clearly indicatedB1 Can still be given if \(y = 3\cos\!\left(\frac{1}{2}x\right)\) graph is incorrect or not attempted. If more than one point marked on the \(y\)-axis then mark the label on the graph intercept.
[3]
Question 9(ii):
AnswerMarks Guidance
Answer/WorkingMarks Guidance
\(\dfrac{\sin(\frac{1}{2}x)}{\cos(\frac{1}{2}x)} = 3\cos\!\left(\tfrac{1}{2}x\right)\)M1 Attempt use of relevant identities to show given equation. Must attempt use of both identities; these must be correct but allow poor notation. e.g. using \(\frac{\sin}{\cos}\!\left(\frac{1}{2}x\right)\) and/or \(3(1-\sin^2)\!\left(\frac{1}{2}x\right)\) could get M1A0.
\(\sin\!\left(\tfrac{1}{2}x\right) = 3\cos^2\!\left(\tfrac{1}{2}x\right)\)
\(\sin\!\left(\tfrac{1}{2}x\right) = 3\!\left(1 - \sin^2\!\left(\tfrac{1}{2}x\right)\right)\)
\(3\sin^2\!\left(\tfrac{1}{2}x\right) + \sin\!\left(\tfrac{1}{2}x\right) - 3 = 0\) AGA1 Obtain given equation, with no errors seen. Use both identities correctly to obtain given equation. Brackets around the \(\frac{1}{2}x\) not required.
\(\sin\!\left(\tfrac{1}{2}x\right) = 0.847,\ -1.18\)M1 Attempt to solve given quadratic to find solution(s) for \(\sin\!\left(\frac{1}{2}x\right)\). Must use quadratic formula (or completing the square) — M0 if attempting to factorise. Allow variables other than \(\sin(\frac{1}{2}x)\), e.g. \(y=\), or even \(x=\). Allow \(-1.18\) to be discarded at any stage.
\(\tfrac{1}{2}x = 1.01,\ 2.13\)M1 Attempt to solve \(\sin\!\left(\frac{1}{2}x\right) = k\). Attempt \(\sin^{-1}\) (their root) and then double the answer.
\(x = 2.02,\ 4.26\)A1 Obtain one correct angle. Allow in degrees (\(116°\) and \(244°\)).
A1Obtain both correct angles, and no others in given range. Must both be in radians (allow equivs as multiples of \(\pi\)). A0 if extra, incorrect angles in given range of \([-2\pi, 2\pi]\) but ignore any outside of given range. SR: if no working shown then allow B1 for each correct solution (max of B1 if in degrees, or extra solns in range).
[6]
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## Question 9(i):

| Answer/Working | Marks | Guidance |
|---|---|---|
| Correct shape for $y = k\cos\!\left(\frac{1}{2}x\right)$ | B1 | Must show intention to pass through $(-\pi, 0)$ and $(\pi, 0)$. Should be roughly symmetrical in the $y$-axis, but condone slightly different $y$-values at $-2\pi$ and $2\pi$. Ignore graph outside of given range. |
| Correct shape for $y = \tan\!\left(\frac{1}{2}x\right)$ | B1 | Must show intention to pass through $(-2\pi, 0)$, $(0, 0)$, $(2\pi, 0)$. Asymptotes need not be marked, but there should be no clear overlap of the limbs, nor significant gaps between them. Ignore graph outside of given range. |
| $(0, 3)$ stated or clearly indicated | B1 | Can still be given if $y = 3\cos\!\left(\frac{1}{2}x\right)$ graph is incorrect or not attempted. If more than one point marked on the $y$-axis then mark the label on the graph intercept. |
| **[3]** | | |

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## Question 9(ii):

| Answer/Working | Marks | Guidance |
|---|---|---|
| $\dfrac{\sin(\frac{1}{2}x)}{\cos(\frac{1}{2}x)} = 3\cos\!\left(\tfrac{1}{2}x\right)$ | M1 | Attempt use of relevant identities to show given equation. Must attempt use of both identities; these must be correct but allow poor notation. e.g. using $\frac{\sin}{\cos}\!\left(\frac{1}{2}x\right)$ and/or $3(1-\sin^2)\!\left(\frac{1}{2}x\right)$ could get M1A0. |
| $\sin\!\left(\tfrac{1}{2}x\right) = 3\cos^2\!\left(\tfrac{1}{2}x\right)$ | | |
| $\sin\!\left(\tfrac{1}{2}x\right) = 3\!\left(1 - \sin^2\!\left(\tfrac{1}{2}x\right)\right)$ | | |
| $3\sin^2\!\left(\tfrac{1}{2}x\right) + \sin\!\left(\tfrac{1}{2}x\right) - 3 = 0$ **AG** | A1 | Obtain given equation, with no errors seen. Use both identities correctly to obtain given equation. Brackets around the $\frac{1}{2}x$ not required. |
| $\sin\!\left(\tfrac{1}{2}x\right) = 0.847,\ -1.18$ | M1 | Attempt to solve given quadratic to find solution(s) for $\sin\!\left(\frac{1}{2}x\right)$. Must use quadratic formula (or completing the square) — M0 if attempting to factorise. Allow variables other than $\sin(\frac{1}{2}x)$, e.g. $y=$, or even $x=$. Allow $-1.18$ to be discarded at any stage. |
| $\tfrac{1}{2}x = 1.01,\ 2.13$ | M1 | Attempt to solve $\sin\!\left(\frac{1}{2}x\right) = k$. Attempt $\sin^{-1}$ (their root) and then double the answer. |
| $x = 2.02,\ 4.26$ | A1 | Obtain one correct angle. Allow in degrees ($116°$ and $244°$). |
| | A1 | Obtain both correct angles, and no others in given range. Must both be in radians (allow equivs as multiples of $\pi$). A0 if extra, incorrect angles in given range of $[-2\pi, 2\pi]$ but ignore any outside of given range. **SR:** if no working shown then allow B1 for each correct solution (max of B1 if in degrees, or extra solns in range). |
| **[6]** | | |

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9 (i) Sketch the graph of $y = \tan \left( \frac { 1 } { 2 } x \right)$ for $- 2 \pi \leqslant x \leqslant 2 \pi$ on the axes provided.\\
On the same axes, sketch the graph of $y = 3 \cos \left( \frac { 1 } { 2 } x \right)$ for $- 2 \pi \leqslant x \leqslant 2 \pi$, indicating the point of intersection with the $y$-axis.\\
(ii) Show that the equation $\tan \left( \frac { 1 } { 2 } x \right) = 3 \cos \left( \frac { 1 } { 2 } x \right)$ can be expressed in the form

$$3 \sin ^ { 2 } \left( \frac { 1 } { 2 } x \right) + \sin \left( \frac { 1 } { 2 } x \right) - 3 = 0$$

Hence solve the equation $\tan \left( \frac { 1 } { 2 } x \right) = 3 \cos \left( \frac { 1 } { 2 } x \right)$ for $- 2 \pi \leqslant x \leqslant 2 \pi$.

\hfill \mbox{\textit{OCR C2 2012 Q9 [9]}}