Exam-Style Problem

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Nov 2017 p31 q3
1823

The equation \(x^3 = 3x + 7\) has one real root, denoted by \(\alpha\).

(i) Show by calculation that \(\alpha\) lies between 2 and 3.

Two iterative formulae, \(A\) and \(B\), derived from this equation are as follows:

\(x_{n+1} = (3x_n + 7)^{\frac{1}{3}}\), \quad (A)

\(x_{n+1} = \frac{x_n^3 - 7}{3}\). \quad (B)

Each formula is used with initial value \(x_1 = 2.5\).

(ii) Show that one of these formulae produces a sequence which fails to converge, and use the other formula to calculate \(\alpha\) correct to 2 decimal places. Give the result of each iteration to 4 decimal places.

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