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June 2020 p31 q6
1873
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 angle AOB is 2x radians. The shaded region is bounded by the tangents AT and BT, and by the minor arc AB. The area of the shaded region is equal to the area of the circle.
(a) Show that x satisfies the equation \(\tan x = \pi + x\).
(b) This equation has one root in the interval \(0 < x < \frac{1}{2}\pi\). Verify by calculation that this root lies between 1 and 1.4.
(c) 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
(a) The length of the tangent from a point to a circle is given by \(AT = r \tan x\) and \(BT = r \tan x\). The area of the sector \(AOB\) is \(\frac{1}{2} r^2 (2x) = r^2 x\). The area of the triangle \(AOT\) 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 circle minus the area of the sector, which is \(\pi r^2 - r^2 x\). Equating the area of the shaded region to the area of the circle, we have:
\(\pi r^2 - r^2 x = r^2 \tan x\)
\(\pi - x = \tan x\)
Thus, \(x\) satisfies \(\tan x = \pi + x\).
(b) Calculate \(\tan 1 \approx 1.5574\) and \(\tan 1.4 \approx 5.7979\). Since \(\pi + 1 \approx 4.1416\) and \(\pi + 1.4 \approx 4.5416\), \(\tan x = \pi + x\) changes sign between 1 and 1.4, confirming a root in this interval.
(c) Using the iterative formula \(x_{n+1} = \arctan(\pi + x_n)\):