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June 2015 p11 q2
1117
The diagram shows the curve \(y = 2x^2\) and the points \(X(-2, 0)\) and \(P(p, 0)\). The point \(Q\) lies on the curve and \(PQ\) is parallel to the \(y\)-axis.
(i) Express the area, \(A\), of triangle \(XPQ\) in terms of \(p\).
(ii) The point \(P\) moves along the \(x\)-axis at a constant rate of 0.02 units per second and \(Q\) moves along the curve so that \(PQ\) remains parallel to the \(y\)-axis. Find the rate at which \(A\) is increasing when \(p = 2\).
Solution
(i) The base of triangle \(XPQ\) is \(2 + p\) and the height is \(2p^2\) (since \(Q\) is on the curve \(y = 2x^2\) at \(x = p\), so \(y = 2p^2\)).