2004 AMC 8 Problem 14

Attempt Problem 14 of the 2004 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 2004 AMC 8 solutions, or check the answer key.

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

14.

What is the area enclosed by the geoboard quadrilateral below?
1515
181218\frac{1}{2}
221222\frac{1}{2}
2727
4141
Answer: C
Concepts:lattice pointarea decomposition
Small hint:
Put the quadrilateral inside an easy surrounding square.
Big hint:
Subtract the outside rectangles and right triangles from the surrounding square.
Solution:
Place the quadrilateral inside the surrounding 10×1010\times10 square, whose area is 100.100.
The five outside pieces have areas 15,15, 12⋅6⋅7=21,\frac12\cdot6\cdot7=21, 12⋅1⋅3=32,\frac12\cdot1\cdot3=\frac32, 12⋅4⋅5=10,\frac12\cdot4\cdot5=10, and 12⋅6⋅10=30.\frac12\cdot6\cdot10=30.
The outside area is 15+21+32+10+30=7712,15+21+\frac32+10+30=77\frac12, so the quadrilateral area is 100−7712=2212.100-77\frac12=22\frac12.
Thus, C is the correct answer.
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