Exam-Style Problems

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June 2002 p3 q4
1819

The sequence of values given by the iterative formula \(x_{n+1} = \frac{2}{3} \left( x_n + \frac{1}{x_n^2} \right)\), with initial value \(x_1 = 1\), converges to \(\alpha\).

(i) Use this formula to find \(\alpha\) correct to 2 decimal places, showing the result of each iteration.

(ii) State an equation satisfied by \(\alpha\), and hence find the exact value of \(\alpha\).

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June 2020 p32 q9
1820

The diagram shows the curves \(y = \cos x\) and \(y = \frac{k}{1+x}\), where \(k\) is a constant, for \(0 \leq x \leq \frac{1}{2}\pi\). The curves touch at the point where \(x = p\).

(a) Show that \(p\) satisfies the equation \(\tan p = \frac{1}{1+p}\).

(b) Use the iterative formula \(p_{n+1} = \arctan\left(\frac{1}{1+p_n}\right)\) to determine the value of \(p\) correct to 3 decimal places. Give the result of each iteration to 5 decimal places.

(c) Hence find the value of \(k\) correct to 2 decimal places.

problem image 1820
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June 2019 p33 q6
1821

The diagram shows the curve \(y = x^4 - 2x^3 - 7x - 6\). The curve intersects the \(x\)-axis at the points \((a, 0)\) and \((b, 0)\), where \(a < b\). It is given that \(b\) is an integer.

  1. Find the value of \(b\).
  2. Hence show that \(a\) satisfies the equation \(a = -\frac{1}{3}(2 + a^2 + a^3)\).
  3. Use an iterative formula based on the equation in part (ii) to determine \(a\) correct to 3 decimal places. Give the result of each iteration to 5 decimal places.
problem image 1821
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Feb/Mar 2019 p32 q2
1822

The sequence of values given by the iterative formula

\(x_{n+1} = \frac{2x_n^6 + 12x_n}{3x_n^5 + 8}\),

with initial value \(x_1 = 2\), converges to \(\alpha\).

(i) Use the formula to calculate \(\alpha\) correct to 4 decimal places. Give the result of each iteration to 6 decimal places. [3]

(ii) State an equation satisfied by \(\alpha\) and hence find the exact value of \(\alpha\). [2]

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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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