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2015 AMC 10B Problem 22

Problem 22 of 25HarderAlgebraGeometry

In the figure shown below, ABCDEABCDE is a regular pentagon and AG=1.AG=1. What is FG+JH+CD?FG + JH + CD?

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Solution

By symmetry, AG=HC=HJ=1AG=HC=HJ=1, and triangles AFGAFG and BGHBGH are congruent, so FG=GHFG=GH. Let FG=bFG=b, and let CD=dCD=d. The similar triangles in the pentagon give 1b=1+b\frac1b=1+b and 1+b1=d.\frac{1+b}{1}=d. The first equation is b2+b1=0b^2+b-1=0, so b=1+52b=\frac{-1+\sqrt{5}}{2}. Then d=1+b=1+52d=1+b=\frac{1+\sqrt{5}}{2}. Therefore FG+JH+CD=b+1+d=1+5. \begin{aligned} FG+JH+CD &= b+1+d \\ &= 1+\sqrt{5}. \end{aligned} Thus, the correct answer is D.

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Concepts: regular polygon · similarity · quadratic

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.