Browsing as Guest. Progress, bookmarks and attempts are disabled.
Log in to track your work.
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.
Solution
(i) Calculate \(f(x) = x^3 - 3x - 7\) at \(x = 2\) and \(x = 3\):
\(f(2) = 2^3 - 3 \times 2 - 7 = 8 - 6 - 7 = -5\)
\(f(3) = 3^3 - 3 \times 3 - 7 = 27 - 9 - 7 = 11\)
Since \(f(2) < 0\) and \(f(3) > 0\), by the Intermediate Value Theorem, \(\alpha\) lies between 2 and 3.