2005 AMC 8 Problem 13

Attempt Problem 13 of the 2005 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 2005 AMC 8 solutions, or check the answer key.

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

13.

The area of polygon ABCDEFABCDEF is 5252 with AB=8,AB=8, BC=9BC=9 and FA=5.FA=5. What is DE+EF?DE+EF?
77
88
99
1010
1111
Answer: C
Concepts:area decompositionrectangle
Small hint:
Complete the figure to a rectangle.
Big hint:
Subtract the polygon area from the rectangle area to get the missing rectangle.
Solution:
Complete the shape to rectangle ABCGABCG, which has area 8⋅9=728\cdot9=72.
The missing rectangle FEDGFEDG has area 72−52=2072-52=20. Its height is DE=BC−FA=9−5=4DE=BC-FA=9-5=4.
Therefore EF=20÷4=5EF=20\div4=5, so DE+EF=4+5=9DE+EF=4+5=9.
Thus, C is the correct answer.
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