2016 AMC 8 Problem 22

Below is the video solution and professionally curated solution for Problem 22 of the 2016 AMC 8, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2016 AMC 8 solutions, or check the answer key.

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Concepts:similaritytriangle area

Difficulty rating: 1640

22.

Rectangle DEFADEFA below is a 3×43 \times 4 rectangle with DC=CB=BA=1.DC=CB=BA=1. The area of the "bat wings" (shaded area) is

2 2

212 2 \dfrac{1}{2}

3 3

312 3 \dfrac{1}{2}

4 4

Video solution:
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Written solution:

Define II to be the midpoint of AD\overline{AD} and GG to be the midpoint of EF.\overline{EF}. Also define HH to be the intersection of CF\overline{CF} and BE.\overline{BE}.

The area of BCE\triangle BCE equals 1214=2.\dfrac{1}{2} \cdot 1 \cdot 4 = 2. By symmetry, we can see that BCH\triangle BCH and EFH\triangle EFH are similar. Since their bases are in a 1:31 : 3 ratio, so are their altitudes. This means that 3IH=HG,3IH = HG, which implies that IH=1.IH = 1.

Therefore, the area of BCH=1211=12.\begin{align*}\triangle BCH &= \dfrac{1}{2} \cdot 1 \cdot 1\\ &= \dfrac{1}{2}.\end{align*} This implies that the area of ECH=212=32.\begin{align*}\triangle ECH &= 2 - \dfrac{1}{2} \\&= \dfrac{3}{2}.\end{align*} Since the figure is symmetric, the total area of the bat wings is 232=3.2 \cdot \dfrac{3}{2} = 3.

Thus, C is the correct answer.

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