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June 2014 p32 q6
1878
In the diagram, A is a point on the circumference of a circle with centre O and radius r. A circular arc with centre A meets the circumference at B and C. The angle OAB is equal to x radians. The shaded region is bounded by AB, AC and the circular arc with centre A joining B and C. The perimeter of the shaded region is equal to half the circumference of the circle.
Show that \(x = \cos^{-1} \left( \frac{\pi}{4 + 4x} \right)\).
Verify by calculation that x lies between 1 and 1.5.
Use the iterative formula \(x_{n+1} = \cos^{-1} \left( \frac{\pi}{4 + 4x_n} \right)\) to determine the value of x correct to 2 decimal places. Give the result of each iteration to 4 decimal places.
Solution
(i) The perimeter of the shaded region is given by the arc length \(r \cdot x\) plus the length of the arc \(2r \cdot \theta\), where \(\theta = \frac{\pi}{2} - x\). The total perimeter is \(r \cdot x + 2r \cdot \left( \frac{\pi}{2} - x \right) = \frac{1}{2} \cdot 2\pi r\). Simplifying gives \(x = \cos^{-1} \left( \frac{\pi}{4 + 4x} \right)\).
(ii) Calculate \(\cos^{-1} \left( \frac{\pi}{4 + 4 \cdot 1} \right) \approx 1.318\) and \(\cos^{-1} \left( \frac{\pi}{4 + 4 \cdot 1.5} \right) \approx 1.047\). Since \(1.047 < x < 1.318\), x lies between 1 and 1.5.