Mathematics 9709 · AS & A Level · Numerical solution of equations

Numerical solution of equations — practice question

The equation $x^3 = 3x + 7$ has a single real root, written as $\alpha$.
(i)[2]

Show by calculation that $\alpha$ is between $2$ and $3$.

(ii)[4]

Two iterative formulae, $A$ and $B$, obtained 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 then applied with initial value $x_1 = 2.5$. Show that one formula generates a sequence that does not converge, and use the other formula to determine $\alpha$ correct to 2 decimal places. Give the result of each iteration to 4 decimal places.

Worked solution & mark scheme

This 6-mark question has a full step-by-step worked solution and mark scheme. One marking point: Calculate the value(s) of a relevant expression or expressions when $x = 2$ and when $x = 3$

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