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Nov 2019 p33 q5
1904
(i) By sketching a suitable pair of graphs, show that the equation \(\ln(x+2) = 4e^{-x}\) has exactly one real root.
(ii) Show by calculation that this root lies between \(x = 1\) and \(x = 1.5\).
(iii) Use the iterative formula \(x_{n+1} = \ln\left( \frac{4}{\ln(x_n + 2)} \right)\) to determine the root correct to 2 decimal places. Give the result of each iteration to 4 decimal places.
Solution
(i) Sketch the graphs of \(y = \ln(x+2)\) and \(y = 4e^{-x}\). The graph of \(y = \ln(x+2)\) is a logarithmic curve starting from \(x = -2\) and increasing. The graph of \(y = 4e^{-x}\) is an exponential decay curve starting from \(y = 4\) when \(x = 0\) and decreasing. These graphs intersect at exactly one point, indicating one real root.
(ii) Calculate \(\ln(1+2) = \ln(3) \approx 1.0986\) and \(4e^{-1} \approx 1.4715\). Since \(\ln(3) < 4e^{-1}\), the function \(\ln(x+2) - 4e^{-x}\) changes sign between \(x = 1\) and \(x = 1.5\), confirming a root in this interval.