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jmc

geometry senior

Problem

is a regular pentagon. , and are the perpendiculars dropped from onto , extended and extended, respectively. Let be the center of the pentagon. If , then find .

problem
Solution
To solve the problem, we compute the area of regular pentagon in two different ways. First, we can divide regular pentagon into five congruent triangles.



If is the side length of the regular pentagon, then each of the triangles , , , , and has base and height 1, so the area of regular pentagon is .

Next, we divide regular pentagon into triangles , , and .



Triangle has base and height . Triangle has base and height . Triangle has base and height . Therefore, the area of regular pentagon is also Hence, which means , or .
Final answer
4