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FM Nov 2023 p11 q07
4184
The curve C has equation \(y = f(x)\), where \(f(x) = \frac{x^2 + 2}{x^2 - x - 2}\).
Find the equations of the asymptotes of C.
Find the coordinates of any stationary points on C, giving your answers correct to 1 decimal place.
Sketch C, stating the coordinates of any intersections with the axes.
Sketch the curve with equation \(y = \frac{1}{f(x)}\).
Find the set of values for which \(\frac{1}{f(x)} < f(x)\).
Solution
(a) The vertical asymptotes occur where the denominator is zero: \(x^2 - x - 2 = 0\). Solving gives \(x = -1\) and \(x = 2\). The horizontal asymptote is found by considering the limits as \(x\) approaches infinity, giving \(y = 1\).
(b) To find stationary points, set \(\frac{dy}{dx} = 0\). The derivative is \(\frac{dy}{dx} = \frac{(x^2 - x - 2)(2x) - (x^2 + 2)(2x - 1)}{(x^2 - x - 2)^2}\). Solving \(x^2 + 8x - 2 = 0\) gives the stationary points \((-8.2, 0.9)\) and \((0.2, -0.9)\).
(c) The sketch shows the curve with asymptotes at \(x = -1\), \(x = 2\), and \(y = 1\). The curve intersects the y-axis at \((0, -1)\).
(d) The sketch of \(y = \frac{1}{f(x)}\) is derived from the sketch in (c), showing the reciprocal relationship.
(e) Solve \(\frac{x^2 + 2}{x^2 - x - 2} = 1\) and \(\frac{x^2 + 2}{x^2 - x - 2} = -1\) to find critical points. The solution gives the intervals \(-4 < x < -1\), \(0 < x < \frac{1}{2}\), and \(x > 2\).