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June 2015 p32 q5
1877
The diagram shows a circle with centre O and radius r. The tangents to the circle at the points A and B meet at T, and the angle AOB is 2x radians. The shaded region is bounded by the tangents AT and BT, and by the minor arc AB. The perimeter of the shaded region is equal to the circumference of the circle.
(i) Show that x satisfies the equation \(\tan x = \pi - x\).
(ii) This equation has one root in the interval \(0 < x < \frac{1}{2}\pi\). Verify by calculation that this root lies between 1 and 1.3.
(iii) Use the iterative formula \(x_{n+1} = \arctan(\pi - x_n)\) to determine the root correct to 2 decimal places. Give the result of each iteration to 4 decimal places.
Solution
(i) The tangents AT and BT are equal in length, and each is equal to \(r \tan x\). The arc length of AB is \(2xr\). The perimeter of the shaded region is \(2r \tan x + 2xr\). This is equal to the circumference of the circle, \(2\pi r\). Therefore, \(2r \tan x + 2xr = 2\pi r\). Dividing by \(2r\), we get \(\tan x + x = \pi\), which rearranges to \(\tan x = \pi - x\).
(ii) Calculate \(\tan 1 \approx 1.5574\) and \(\tan 1.3 \approx 3.6021\). For \(x = 1\), \(\tan 1 - (\pi - 1) \approx 1.5574 - 2.1416 = -0.5842\). For \(x = 1.3\), \(\tan 1.3 - (\pi - 1.3) \approx 3.6021 - 1.8416 = 1.7605\). The sign change indicates a root between 1 and 1.3.
(iii) Using the iterative formula \(x_{n+1} = \arctan(\pi - x_n)\):