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Nov 2018 p32 q5
1829
The equation of a curve is \(y = x \ln(8 - x)\). The gradient of the curve is equal to 1 at only one point, when \(x = a\).
(i) Show that \(a\) satisfies the equation \(x = 8 - \frac{8}{\ln(8 - x)}\).
(ii) Verify by calculation that \(a\) lies between 2.9 and 3.1.
(iii) Use an iterative formula based on the equation in part (i) to determine \(a\) correct to 2 decimal places. Give the result of each iteration to 4 decimal places.
Solution
(i) Differentiate \(y = x \ln(8 - x)\) using the product rule: \(\frac{dy}{dx} = \ln(8 - x) - \frac{x}{8 - x}\).
Set \(\frac{dy}{dx} = 1\) and solve: \(\ln(8 - x) - \frac{x}{8 - x} = 1\).
Rearrange to find \(x = 8 - \frac{8}{\ln(8 - x)}\).
(ii) Calculate \(8 - \frac{8}{\ln(8 - 2.9)} = 3.09\) and \(8 - \frac{8}{\ln(8 - 3.1)} = 2.97\).
Since \(3.09 > 2.9\) and \(2.97 < 3.1\), \(a\) lies between 2.9 and 3.1.
(iii) Use the iterative formula \(x_{n+1} = 8 - \frac{8}{\ln(8 - x_n)}\).