2002 AIME I Problem 10

Below is the professionally curated solution for Problem 10 of the 2002 AIME I, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2002 AIME I solutions, or check the answer key.

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Concepts:angle bisector theoremarea ratioPythagorean Theorem

Difficulty rating: 2720

10.

In the diagram below, angle ABCABC is a right angle. Point DD is on BC,\overline{BC}, and AD\overline{AD} bisects angle CAB.CAB. Points EE and FF are on AB\overline{AB} and AC,\overline{AC}, respectively, so that AE=3AE = 3 and AF=10.AF = 10. Given that EB=9EB = 9 and FC=27,FC = 27, find the integer closest to the area of quadrilateral DCFG.DCFG.

Solution:

Here AB=3+9=12,AB = 3 + 9 = 12, AC=10+27=37,AC = 10 + 27 = 37, and angle BB is right, so BC=372122=35BC = \sqrt{37^2 - 12^2} = 35 and [ABC]=121235=210.[ABC] = \frac{1}{2} \cdot 12 \cdot 35 = 210. The quadrilateral is triangle ADCADC with triangle AGFAGF removed, where GG is the intersection of AD\overline{AD} and EF.\overline{EF}.

By the angle bisector theorem in triangle ABC,ABC, BD:DC=AB:AC=12:37,BD : DC = AB : AC = 12 : 37, so [ADC]=3749210=11107.[ADC] = \frac{37}{49} \cdot 210 = \frac{1110}{7}. In triangle AEF,AEF, ray AGAG bisects the same angle, so EG:GF=AE:AF=3:10EG : GF = AE : AF = 3 : 10 and [AGF]=1013[AEF].[AGF] = \frac{10}{13}\,[AEF]. Also [AEF]=AEABAFAC[ABC]=3121037210=52537.[AEF] = \frac{AE}{AB} \cdot \frac{AF}{AC}\,[ABC] = \frac{3}{12} \cdot \frac{10}{37} \cdot 210 = \frac{525}{37}.

Therefore [DCFG]=11107101352537=111075250481158.5710.92=147.66,[DCFG] = \frac{1110}{7} - \frac{10}{13} \cdot \frac{525}{37} = \frac{1110}{7} - \frac{5250}{481} \approx 158.57 - 10.92 = 147.66, and the closest integer is 148.148.

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