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Feb/Mar 2021 p32 q9
1844
Let \(f(x) = \frac{e^{2x} + 1}{e^{2x} - 1}\), for \(x > 0\).
(a) The equation \(x = f(x)\) has one root, denoted by \(a\). Verify by calculation that \(a\) lies between 1 and 1.5. [2]
(b) Use an iterative formula based on the equation in part (a) to determine \(a\) correct to 2 decimal places. Give the result of each iteration to 4 decimal places. [3]
(c) Find \(f'(x)\). Hence find the exact value of \(x\) for which \(f'(x) = -8\). [6]
Solution
(a) Calculate \(f(1) = \frac{e^{2} + 1}{e^{2} - 1}\) and \(f(1.5) = \frac{e^{3} + 1}{e^{3} - 1}\). Verify that \(f(1) < 1\) and \(f(1.5) > 1.5\), confirming \(a\) is between 1 and 1.5.
(b) Use the iterative formula \(x_{n+1} = \frac{e^{2x_n} + 1}{e^{2x_n} - 1}\). Start with \(x_1 = 1\):
\(x_2 = 1.2311\)
\(x_3 = 1.2032\)
\(x_4 = 1.2002\)
\(x_5 = 1.2000\)
Thus, \(a = 1.20\) to 2 decimal places.
(c) Use the quotient rule to find \(f'(x) = \frac{(e^{2x} - 1)(2e^{2x}) - (e^{2x} + 1)(2e^{2x})}{(e^{2x} - 1)^2}\).
Simplify to \(f'(x) = \frac{-4e^{2x}}{(e^{2x} - 1)^2}\).