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Printjmc
geometry senior
Problem
Convex pentagon has side lengths , , and . Moreover, the pentagon has an inscribed circle (a circle tangent to each side of the pentagon). Find the area of .
Solution
Assume the incircle touches , , , , at respectively. Then let , , . So we have , and =7, solve it we have , , . Let the center of the incircle be , by SAS we can proof triangle is congruent to triangle , and triangle is congruent to triangle . Then we have , . Extend , cross ray at , ray at , then by AAS we have triangle is congruent to triangle . Thus . Let , then . So by law of cosine in triangle and triangle we can obtain, solved it gives us , which yield triangle to be a triangle with side length 15, 15, 24, draw a height from to divides it into two triangles with side lengths 9, 12, 15, so the area of triangle is 108. Triangle is a triangle with side lengths 6, 8, 10, so the area of two of them is 48, so the area of pentagon is .
Final answer
60