Medians \(\overline{AX}\) and \(\overline{BY}\) of \(\triangle{ABC}\) are perpendicular at point \(G\). Prove that \(AB=CG\).


In your diagram, \(\angle{AGB}\) should appear to be a right angle.

benjamingu22  Sep 30, 2017

Here's an  image :




Bisect angle ACB.....and this will form altitude CE


And AC = BC    And angle ACG  = angle BCG    And  CE is common

So by SAS triangle ACG is congruent to triangle BCG

Then AG = BG  so triangle AGB is isosceles....and because AGB is right.......then angles ABG and BAG  =  45°

And since CEB is right and ABG = 45°....then angle BGE is also = 45°

Then triangle  BGE is also isosceles  with EB = EG


Draw  XY......and by hypotenuse-leg, triangle AYG is congruent to triangle BXG


So......AY  =  BX.....so  XY is parallel to AB


And because BX splits BC equally.......then  CD  =  ED


And triangle CDX  is similar to triangle CEB

And  since CD  is (1/2)  of CE then   DX =  (1/2)EB =  (1/2) EG


And angle XAB = angle YXG  =  45°  and since GDX is right, then angle XGD = 45°


So triangles  GDX and  GEB   are similar


And because triangle GDX is isosceles.....then DX = DG

Thus   DX  = (1/2)EG   and by substitution  DG = (1/2) EG  ⇒ 2DG = EG = EB




EB + DG  =  3DG

ED  =  3DG = CD

Add  DG to everything  

ED + DG  = 4DG  = CD + DG

4DG  =  2 EG  =  CG


But  EG =  EB   ....so....

2EB  = CG

But 2EB  = AB


So....  AB =  CG  




cool cool cool

CPhill  Oct 1, 2017
edited by CPhill  Oct 1, 2017

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