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Nov 2023 p32 q6
1879
(a) By sketching a suitable pair of graphs, show that the equation \(\cot x = 2 - \cos x\) has one root in the interval \(0 < x \leq \frac{1}{2}\pi\).
(b) Show by calculation that this root lies between 0.6 and 0.8.
(c) Use the iterative formula \(x_{n+1} = \arctan\left( \frac{1}{2 - \cos x_n} \right)\) to determine the root correct to 2 decimal places. Give the result of each iteration to 4 decimal places.
Solution
(a) Sketch the graphs of \(y = \cot x\) and \(y = 2 - \cos x\) for \(0 < x \leq \frac{1}{2}\pi\). The intersection point of these graphs indicates the root of the equation \(\cot x = 2 - \cos x\). The graph shows one intersection in the given interval.
(b) Calculate \(\cot(0.6)\) and \(\cot(0.8)\) and compare with \(2 - \cos(0.6)\) and \(2 - \cos(0.8)\). For \(x = 0.6\), \(\cot(0.6) \approx 1.46\) and \(2 - \cos(0.6) \approx 1.34\). For \(x = 0.8\), \(\cot(0.8) \approx 1.03\) and \(2 - \cos(0.8) \approx 1.29\). The change in sign between these values indicates a root between 0.6 and 0.8.
(c) Use the iterative formula \(x_{n+1} = \arctan\left( \frac{1}{2 - \cos x_n} \right)\) starting with an initial guess, such as \(x_0 = 0.7\). Perform iterations: