2023 ACJC Promo Q7

2023 ACJC Promo Q7

10 marks
Promo

The curve \(C\) has parametric equations

\(x=4\cos \theta \),       \(y=3\sin \theta \),        \(0<\theta \le \pi \).

  1. Point \(Q\) lies on \(C\) and is moving along \(C\), such that its x coordinate is increasing at the constant rate of \(15\) units per second. Determine the exact rate of change of \(\theta \) at the instant when the coordinates of \(Q\) is \(\left( 2,\frac{3\sqrt{3}}{2} \right)\).[3]

It is given that point \(P\) also lies on \(C\) and has parameter \(p\) where \(0 < p < \frac{\pi }{2}\).

  1. Show that the equation of the tangent to \(C\) at the point \(P\), is given by
    \(y=\left( -\frac{3}{4}\cot p \right)x+3\,\mathrm{cosec}\,p\).[3]
  2. The tangent to the curve at \(P\) meets the \(x\)-axis at the point \(A\) and the \(y\)-axis at the point \(B\). Find the area of the triangle \(OAB\).[2]
  3. Let \(M\) be the mid-point of \(AB\). Find a cartesian equation of the curve traced by \(M\) as \(p\) varies.[2]

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Answer:(a) \(\frac{\mathrm d\theta}{\mathrm dt}=-\frac{5\sqrt3}{2}\text{ rad s}^{-1}\). (b) \(y=-\frac34(\cot p)x+3\csc p\). (c) \(6\sec p\csc p\text{ units}^2\). (d) \(\frac4{x^2}+\frac9{4y^2}=1\), \(x>2\), \(y>\frac32\).

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