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Mock AIME 4 2006-2007 Problems/Problem 4

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Problem

Points A, B, and C are on the circumference of a unit circle so that the measure of \widehat{AB} is 72^{\circ}, the measure of \widehat{BC} is 36^{\circ}, and the measure of \widehat{AC} is 108^\circ. The area of the triangular shape bounded by \widehat{BC} and line segments \overline{AB} and \overline{AC} can be written in the form \frac{m}{n} \cdot \pi, where m and n are relatively prime positive integers. Find m + n.

Solution

Image:2007_4_MockAIME-4.png

Let the center of the circle be O. The area of the desired region is easily seen to be that of sector BOC plus the area of triangle AOB minus the area of triangle AOC. Using the area formula K_{\triangle XYZ} = \frac{1}{2} XY \cdot YZ \cdot \sin Y to compute the areas of the two triangles, this is \pi \cdot \frac{36}{360} + \frac{1}{2}\sin 72^\circ - \frac{1}{2}\sin108^{\circ} = \frac{1}{10}\cdot \pi, so the answer is 1 + 10 = 011.

See also

Mock AIME 4 2006-2007 (Problems, Source)
Preceded by
Problem 3
Followed by
Problem 5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
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