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imc

geometry intermediate

Problem

In the figure shown below, is a regular pentagon and . What is ?
(A)
(B)
(C)
(D)
Solution
Triangle is isosceles, so . since is also isosceles. Using the symmetry of pentagon , notice that . Therefore, . Since , So, since must be greater than 0. Notice that . Therefore, Note by Fasolinka: Alternatively, extend and call its intersection with . It is not hard to see that quadrilaterals and are parallelograms, so , and the same result is achieved.
Final answer
D