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June 2006 p3 q6
1894
(i) By sketching a suitable pair of graphs, show that the equation \(2 \cot x = 1 + e^x\), where \(x\) is in radians, has only one root in the interval \(0 < x < \frac{1}{2} \pi\).
(ii) Verify by calculation that this root lies between 0.5 and 1.0.
(iii) Show that this root also satisfies the equation \(x = \arctan\left(\frac{2}{1 + e^x}\right)\).
(iv) Use the iterative formula \(x_{n+1} = \arctan\left(\frac{2}{1 + e^{x_n}}\right)\), with initial value \(x_1 = 0.7\), to determine this root correct to 2 decimal places. Give the result of each iteration to 4 decimal places.
Solution
(i) Sketch the graphs of \(y = 2 \cot x\) and \(y = 1 + e^x\). The intersection of these graphs in the interval \(0 < x < \frac{1}{2} \pi\) indicates the root. The behavior of \(2 \cot x\) and \(1 + e^x\) suggests only one intersection point in this interval.
(ii) Evaluate \(2 \cot x - 1 - e^x\) at \(x = 0.5\) and \(x = 1.0\). The sign change between these points confirms a root exists between them.
(iii) Rearrange the original equation to show it is equivalent to \(x = \arctan\left(\frac{2}{1 + e^x}\right)\).