2017 AMC 8 Problem 16

Attempt Problem 16 of the 2017 AMC 8 below, then check your answer against the video solution and professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2017 AMC 8 solutions, or check the answer key.

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

16.

In the figure shown below, choose point DD on side BCBC so that △ACD\triangle ACD and △ABD\triangle ABD have equal perimeters. What is the area of △ABD?\triangle ABD?
34 \dfrac{3}{4}
32 \dfrac{3}{2}
2 2
125 \dfrac{12}{5}
52 \dfrac{5}{2}
Answer: D
Concepts:area ratioperimeter
Small hint:
Use the equal-perimeter condition to find BDBD and CDCD.
Big hint:
Once BCBC is split, the two triangles have the same altitude from AA, so compare areas by base lengths.
Video solution:
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Written solution:
The only way to split BC‾\overline{BC} into two parts such that the two triangles have the same perimeter is if CD‾=3\overline{CD} = 3 and BD‾=2.\overline{BD} = 2.
△ACD\triangle ACD and △ABD\triangle ABD have the same altitudes, so their areas are proportional to their bases. This means that the area of △ABD\triangle ABD is 25\dfrac{2}{5} the area of △ABC,\triangle ABC, which is 25⋅3⋅42=125.\dfrac{2}{5} \cdot 3 \cdot \dfrac{4}{2} = \dfrac{12}{5}.
Thus, D is the correct answer.
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