Posted by TUHITUHI on Thursday, August 16, 2012 at 9:15am.
given:
ABC is an isosceles triangle with
AB = AC ---- by definition
If AD is the median to BC
BD = CD ---- given
AD = AD --- common side
therefore triangle ABD is congruent to triangle ACD (SSS)
Therefore angle ADB = angle ADC
but angle ADB + ADC = 180
so each angle = 90°
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