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    Question

    In a triangle △ ABC, AB = AC and ∠A = 80°, then

    find ∠B.
    A 50° Correct Answer Incorrect Answer
    B 80° Correct Answer Incorrect Answer
    C 40° Correct Answer Incorrect Answer
    D 45° Correct Answer Incorrect Answer

    Solution

    In triangle △ABC, AB = AC, so it's an isosceles triangle.
    Given ∠A = 80°, the base angles ∠B and ∠C are equal. Using the angle sum of a triangle:
    ∠A + ∠B + ∠C = 180°
    80° + ∠B + ∠B = 180°
    2∠B = 100°
    ∠B = 50°

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