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June 2011 p32 q4
1865
The diagram shows a semicircle ACB with centre O and radius r. The tangent at C meets AB produced at T. The angle BOC is x radians. The area of the shaded region is equal to the area of the semicircle.
(i) Show that x satisfies the equation \(\tan x = x + \pi\).
(ii) Use the iterative formula \(x_{n+1} = \arctan(x_n + \pi)\) to determine x correct to 2 decimal places. Give the result of each iteration to 4 decimal places.
Solution
(i) The length of the tangent CT can be expressed as \(CT = r \tan x\). The area of the semicircle is \(\frac{1}{2} \pi r^2\). The area of the sector BOC is \(\frac{1}{2} r^2 x\). The area of the triangle OCT is \(\frac{1}{2} r \cdot r \tan x = \frac{1}{2} r^2 \tan x\). The area of the shaded region is the area of the triangle minus the area of the sector: \(\frac{1}{2} r^2 \tan x - \frac{1}{2} r^2 x\). Setting this equal to the area of the semicircle gives:
\(\frac{1}{2} r^2 \tan x - \frac{1}{2} r^2 x = \frac{1}{2} \pi r^2\)
Dividing through by \(\frac{1}{2} r^2\) gives:
\(\tan x - x = \pi\)
Thus, \(\tan x = x + \pi\).
(ii) Using the iterative formula \(x_{n+1} = \arctan(x_n + \pi)\), start with an initial guess, say \(x_0 = 1\).
1. \(x_1 = \arctan(1 + \pi) = 1.3521\)
2. \(x_2 = \arctan(1.3521 + \pi) = 1.3490\)
3. \(x_3 = \arctan(1.3490 + \pi) = 1.3492\)
4. \(x_4 = \arctan(1.3492 + \pi) = 1.3492\)
The value converges to 1.35 when rounded to 2 decimal places.