在△ABC中,∠A=80°,当∠B=时,$\triangle A B C$是等腰三角形.
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在$\triangle A B C$中,$\angle A = 80 ^ {\circ}$,当$\angle B = \angle A = 80 ^ {\circ}$时,$\triangle A B C$是等腰三角形;当$\angle C = \angle A = 80 ^ {\circ}$时,$\triangle A B C$是等腰三角形,此时$\angle B = 180 ^ {\circ} - \angle A - \angle C =$$20 ^ {\circ}$. 所以当$\angle B = 20 ^ {\circ}$或$80 ^ {\circ}$时,$\triangle A B C$是等腰三角形,故答案为$20 ^ {\circ}$或$80 ^ {\circ}$.