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June 2020 p33 q6
1902
(a) By sketching a suitable pair of graphs, show that the equation \(x^5 = 2 + x\) has exactly one real root.
(b) Show that if a sequence of values given by the iterative formula \(x_{n+1} = \frac{4x_n^5 + 2}{5x_n^4 - 1}\) converges, then it converges to the root of the equation in part (a).
(c) Use the iterative formula with initial value \(x_1 = 1.5\) to calculate the root correct to 3 decimal places. Give the result of each iteration to 5 decimal places.
Solution
(a) To show that the equation \(x^5 = 2 + x\) has exactly one real root, sketch the graphs of \(y = x^5\) and \(y = x + 2\). The intersection of these graphs represents the solution to the equation. Since \(x^5\) is a strictly increasing function and \(x + 2\) is a linear function, they intersect at exactly one point, confirming one real root.
(b) Consider the iterative formula \(x_{n+1} = \frac{4x_n^5 + 2}{5x_n^4 - 1}\). If this sequence converges, it must satisfy the equation \(x = \frac{4x^5 + 2}{5x^4 - 1}\). Rearranging gives \(x^5 = 2 + x\), which is the equation from part (a). Therefore, if the sequence converges, it converges to the root of the equation.
(c) Using the iterative formula with \(x_1 = 1.5\):