As shown in the figure, in △ ABC, ab = AC, ∠ BAC = 120 °, point m is on edge BC, am = BM

As shown in the figure, in △ ABC, ab = AC, ∠ BAC = 120 °, point m is on edge BC, am = BM

It is proved that: ∵ in △ ABC, ab = AC, ∠ BAC = 120 °, ∵ B = ∠ C = 30 °, ∵ am = BM, ∵ mAb = ∠ B = 30 °, ∵ MAC = 120 ° - 30 ° = 90 °, ∵ C = 30 °, ∵ cm = 2am = 2bm