0606 P12 - Nov 2025 - Q13 - 4 marks
7090
For \(-1\leqslant x\leqslant1\), it is given that \(x=2-3\sin\theta\) and \(y=3\cot^2\theta\).
Find \(y\) in terms of \(x\).
Solution
Answer: \(y=3\left(\left(\frac{3}{2-x}\right)^2-1\right)\).
We are given
\(x=2-3\sin\theta.\)
Rearrange this to express \(\sin\theta\) in terms of \(x\):
\(3\sin\theta=2-x,\)
so
\(\sin\theta=\dfrac{2-x}{3}.\)
Also,
\(y=3\cot^2\theta.\)
Using
\(\cot^2\theta=\dfrac{\cos^2\theta}{\sin^2\theta}=\dfrac{1-\sin^2\theta}{\sin^2\theta},\)
we get
\(y=3\left(\dfrac{1-\sin^2\theta}{\sin^2\theta}\right).\)
Substitute \(\sin\theta=\dfrac{2-x}{3}\):
\(y=3\left(\dfrac{1-\left(\dfrac{2-x}{3}\right)^2}{\left(\dfrac{2-x}{3}\right)^2}\right).\)
This is the same as
\(y=3\left(\dfrac{1}{\left(\dfrac{2-x}{3}\right)^2}-1\right).\)
Therefore
\(y=3\left(\left(\dfrac{3}{2-x}\right)^2-1\right).\)