In the isosceles triangle ABC, ab = AC, ad is the middle line on the bottom edge

From the question, ab = AC, angle abd = angle ACD, ad is the middle line on the bottom edge, so BD = DC
From this we can get that the triangle abd is congruent with the triangle ADC, so the angle ADB = angle ADC,
Angle ADB = angle ADC = 90 ° i.e. ad ⊥ BC

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