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In the isosceles triangle ABC, if angle a is equal to 30 degrees, then how many degrees is angle B
① If ∠ A is the top angle, then ∠ B ＋ ∠ C = 180 ° - ∠ a ∫ B = ∠ C ∫ B = (180 ° - 30 °) / 2 = 75 ° ② ∠ A is the bottom angle, if ∠ B is also the bottom angle, then ∠ a = ∠ B = 30 ° ③ ∠ A is the bottom angle, if ∠ B is the top angle, if ∫ C is the bottom angle, then ∠ B = 180 ° - ∠ a ∫ C ∫ a = ∠ C ∫ B = 180-2 ∫ a = 180 ° - 60 ° = 120 ° (this topic uses the knowledge that the inner angle of triangle is equal to 180 ° and the bottom angle of isosceles triangle)
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