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Problem 186
186
The diagram shows a semicircle ABC with centre O and radius 8 cm. Angle AOB = \(\theta\) radians.
(i) In the case where \(\theta = 1\), calculate the area of the sector BOC.
(ii) Find the value of \(\theta\) for which the perimeter of sector AOB is one half of the perimeter of sector BOC.
(iii) In the case where \(\theta = \frac{1}{3}\pi\), show that the exact length of the perimeter of triangle ABC is \((24 + 8\sqrt{3})\) cm.
Solution
(i) For \(\theta = 1\), angle BOC = \(\pi - \theta\). The area of sector BOC is given by \(\frac{1}{2} r^2 (\pi - \theta)\). Substituting \(r = 8\) and \(\theta = 1\), the area is \(\frac{1}{2} \times 8^2 \times (\pi - 1) = 32(\pi - 1)\) or approximately 68.5.
(ii) The perimeter of sector AOB is \(8 + 8 + 8\theta\) and the perimeter of sector BOC is \(8 + 8 + 8(\pi - \theta)\). Setting \(8 + 8 + 8\theta = \frac{1}{2}(8 + 8 + 8(\pi - \theta))\), solve for \(\theta\):
\(16 + 8\theta = 8 + 8 + 4\pi - 4\theta\)
\(12\theta = 4\pi - 16\)
\(\theta = \frac{1}{3}(\pi - 2)\) or approximately 0.381.
(iii) For \(\theta = \frac{1}{3}\pi\), AB = 8 cm. Using the cosine rule or trigonometry, BC = \(2 \times 8 \sin \frac{\pi}{3} = 8\sqrt{3}\). The perimeter of triangle ABC is \(8 + 8 + 8\sqrt{3} = 24 + 8\sqrt{3}\) cm.