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Geometry Difficulty 3.8 AMC 10/12 Prove it Japan

ABCABC is a triangle and DD, EE, FF are midpoints of BCBC, CACA, ABAB respectively. If AD=3AD = 3, BE=4BE = 4 and CF=5CF = 5, what is the area of ABCABC?

Solution

Let PQR|PQR| denote the area of triangle PQRPQR. ADAD, BEBE, CFCF cross at a point GG and AG:GD=BG:GE=CG:GF=2:1AG : GD = BG : GE = CG : GF = 2 : 1. Take a point CC' on line GCGC so that GG is the midpoint of CCCC'. From CG=GCC'G = GC and CG:GF=2:1CG : GF = 2 : 1, it follows that CF=GFC'F = GF. Since AF=BFAF = BF, CF=GFC'F = GF and AFC=BFG\angle AFC' = \angle BFG, triangles AFCAFC' and BFGBFG are congruent and so AC=BGAC' = BG. Now we have AC=BG=83AC' = BG = \frac{8}{3}, AG=2AG = 2, GC=GC=103GC' = GC = \frac{10}{3}, so AGCAGC' is a right triangle with hypotenuse GCGC'. Therefore AGC=83|AGC'| = \frac{8}{3}. By CG=GCC'G = GC we get AGC=AGC=83|AGC| = |AGC'| = \frac{8}{3}, and by BG:GE=2:1BG : GE = 2 : 1 we get ABC=3AGC=8|ABC| = 3|AGC| = 8.

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