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Japanese Junior Mathematical Olympiad

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 C′C' on line GCGC so that GG is the midpoint of CC′CC'. From C′G=GCC'G = GC and CG:GF=2:1CG : GF = 2 : 1, it follows that C′F=GFC'F = GF. Since AF=BFAF = BF, C′F=GFC'F = GF and ∠AFC′=∠BFG\angle AFC' = \angle BFG, triangles AFC′AFC' 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 AGC′AGC' is a right triangle with hypotenuse GC′GC'. Therefore ∣AGC′∣=83|AGC'| = \frac{8}{3}. By C′G=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∣=3∣AGC∣=8|ABC| = 3|AGC| = 8.

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