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Nov 2005 p3 q4
1817
The equation \(x^3 - x - 3 = 0\) has one real root, \(\alpha\).
(i) Show that \(\alpha\) lies between 1 and 2.
Two iterative formulae derived from this equation are as follows:
\(x_{n+1} = x_n^3 - 3, \quad (A)\)
\(x_{n+1} = (x_n + 3)^{\frac{1}{3}}, \quad (B)\)
Each formula is used with initial value \(x_1 = 1.5\).
(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.
Solution
(i) To show that \(\alpha\) lies between 1 and 2, evaluate the function \(f(x) = x^3 - x - 3\) at \(x = 1\) and \(x = 2\):
\(f(1) = 1^3 - 1 - 3 = -3\)
\(f(2) = 2^3 - 2 - 3 = 3\)
Since \(f(1) < 0\) and \(f(2) > 0\), by the Intermediate Value Theorem, there is a root \(\alpha\) between 1 and 2.
(ii) Apply the iterative formulae:
Using formula (A):
\(x_1 = 1.5\)
\(x_2 = (1.5)^3 - 3 = 0.375\)
\(x_3 = (0.375)^3 - 3 = -2.9473\)
The sequence diverges.
Using formula (B):
\(x_1 = 1.5\)
\(x_2 = (1.5 + 3)^{\frac{1}{3}} = 1.6509\)
\(x_3 = (1.6509 + 3)^{\frac{1}{3}} = 1.6684\)
\(x_4 = (1.6684 + 3)^{\frac{1}{3}} = 1.6715\)
\(x_5 = (1.6715 + 3)^{\frac{1}{3}} = 1.6719\)
The sequence converges to \(\alpha = 1.67\) correct to 2 decimal places.