ABC is a triangle and D, E, F are midpoints of BC, CA, AB respectively. If AD=3, BE=4 and CF=5, what is the area of ABC?
Solution
Let ∣PQR∣ denote the area of triangle PQR. AD, BE, CF cross at a point G and AG:GD=BG:GE=CG:GF=2:1. Take a point C′ on line GC so that G is the midpoint of CC′. From C′G=GC and CG:GF=2:1, it follows that C′F=GF. Since AF=BF, C′F=GF and ∠AFC′=∠BFG, triangles AFC′ and BFG are congruent and so AC′=BG. Now we have AC′=BG=38, AG=2, GC′=GC=310, so AGC′ is a right triangle with hypotenuse GC′. Therefore ∣AGC′∣=38. By C′G=GC we get ∣AGC∣=∣AGC′∣=38, and by BG:GE=2:1 we get ∣ABC∣=3∣AGC∣=8.
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