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Problem 210
210
The diagram shows a circle with centre O and radius r cm. Points A and B lie on the circle and angle AOB = 2\theta radians. The tangents to the circle at A and B meet at T.
(i) Express the perimeter of the shaded region in terms of r and \theta.
(ii) In the case where r = 5 and \theta = 1.2, find the area of the shaded region.
Solution
(i) The arc length AB is given by \(2r\theta\). The tangent segments AT and BT are equal, and each is \(r \tan \theta\) because \(\tan \theta = \frac{AT}{r}\). Therefore, the perimeter \(P\) of the shaded region is:
\(P = 2r\theta + 2r \tan \theta\)
(ii) For r = 5 and \theta = 1.2, the area of triangle AOT is \(\frac{1}{2} \times 5 \times 5 \times \tan 1.2\). The area of sector AOB is \(\frac{1}{2} \times 5^2 \times 2.4\). The shaded area is twice the area of triangle AOT minus the area of sector AOB: