Centroid 80

Geometry Level 3

Evaluate A G AG if G G is the centroid of triangle A B C ABC


The answer is 3.6055512755.

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1 solution

X X
Jul 3, 2018

Let C G \overline{CG} intersects A B \overline{AB} at D D ,then D G = 2 , D B 2 = 3 2 + 2 2 2 × 3 × 2 × cos 6 0 = 7 \overline{DG}=2,\overline{DB}^2=3^2+2^2-2\times3\times2\times\cos60^\circ=7

So B D 2 + D G 2 2 B D × D G cos B D G = B G 2 = 9 \overline{BD}^2+\overline{DG}^2-2\overline{BD}\times\overline{DG}\cos\angle BDG=\overline{BG}^2=9 A G 2 = D A 2 + D G 2 2 D A × D G cos A D G = B D 2 + D G 2 + 2 B D × D G cos B D G = 13 \overline{AG}^2=\overline{DA}^2+\overline{DG}^2-2\overline{DA}\times\overline{DG}\cos\angle ADG=\overline{BD}^2+\overline{DG}^2+2\overline{BD}\times\overline{DG}\cos\angle BDG=13

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