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smc

geometry senior

Problem

Triangle is equilateral with side length . Suppose that is the center of the inscribed circle of this triangle. What is the area of the circle passing through , , and ?
(A)
(B)
(C)
(D)
Solution
Construct the circle that passes through , , and , centered at . Also notice that and are the angle bisectors of angle and respectively. We then deduce . Consider another point on Circle opposite to point . As is an inscribed quadrilateral of Circle , . Afterward, deduce that . By the Cosine Rule, we have the equation: (where is the radius of circle ) The area is therefore .
Final answer
B