2008 AMC 8 Problem 23

Attempt Problem 23 of the 2008 AMC 8 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2008 AMC 8 solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

23.

In square ABCE,ABCE, AF=2FEAF=2FE and CD=2DE.CD=2DE. What is the ratio of the area of △BFD\triangle BFD to the area of square ABCE?ABCE?
 16 \ \dfrac{1}{6}
 29 \ \dfrac{2}{9}
 518 \ \dfrac{5}{18}
 13 \ \dfrac{1}{3}
 720 \ \dfrac{7}{20}
Answer: C
Concepts:area decompositiontriangle areasquare (geometry)
Small hint:
Because only a ratio is needed, choose a convenient side length for the square.
Big hint:
Take the side length as 33 so the split lengths are 22 and 1.1.
Solution:
Because the answer is a ratio, choose the side length of the square to be 33. Then AF=2AF=2, FE=1FE=1, CD=2CD=2, and DE=1DE=1.
The square has area 99. The areas of triangles ABFABF and BCDBCD are each 12⋅3⋅2=3\dfrac{1}{2}\cdot3\cdot2=3.
Triangle DEFDEF has area 12⋅1⋅1=12\dfrac{1}{2}\cdot1\cdot1=\dfrac{1}{2}.
So [BFD]=9−3−3−12=52[BFD]=9-3-3-\dfrac{1}{2}=\dfrac{5}{2}, and the desired ratio is 529=518\dfrac{\frac{5}{2}}{9}=\dfrac{5}{18}.
Thus, C is the correct answer.
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