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June 2008 p3 q8
2310
In the diagram the tangent to a curve at a general point \(P\) with coordinates \((x, y)\) meets the \(x\)-axis at \(T\). The point \(N\) on the \(x\)-axis is such that \(PN\) is perpendicular to the \(x\)-axis. The curve is such that, for all values of \(x\) in the interval \(0 < x < \frac{1}{2}\pi\), the area of triangle \(PTN\) is equal to \(\tan x\), where \(x\) is in radians.
(i) Using the fact that the gradient of the curve at \(P\) is \(\frac{PN}{TN}\), show that \(\frac{dy}{dx} = \frac{1}{2}y^2 \cot x\).
(ii) Given that \(y = 2\) when \(x = \frac{1}{6}\pi\), solve this differential equation to find the equation of the curve, expressing \(y\) in terms of \(x\).
Solution
(i) The gradient of the curve at \(P\) is given by \(\frac{PN}{TN} = \frac{y}{TN}\). The area of triangle \(PTN\) is \(\frac{1}{2} \times TN \times y = \tan x\). Therefore, \(TN = \frac{2 \tan x}{y}\). The gradient \(\frac{dy}{dx} = \frac{y}{TN} = \frac{y^2}{2 \tan x} = \frac{1}{2}y^2 \cot x\).
(ii) Separate variables: \(\frac{dy}{y^2} = \frac{1}{2} \cot x \, dx\). Integrate both sides: \(-\frac{1}{y} = \frac{1}{2} \ln(\sin x) + C\). Given \(y = 2\) when \(x = \frac{1}{6}\pi\), substitute to find \(C\): \(-\frac{1}{2} = \frac{1}{2} \ln(\sin \frac{1}{6}\pi) + C\). Solve for \(C\) and substitute back to find \(y\): \(y = \frac{2}{1 - \ln(2 \sin x)}\).