In an isosceles triangle ABC with base AC, the outside angle at apex A is 140. Find the angle B of the triangle.

univerkov | April 30, 2021 | education

∠ ОАВ = 140 degrees (by condition);

∠ OAB + ∠ BAC = 180 degrees (adjacent angles);

∠ BАС = 180 – 140 = 40 degrees;

∠ BCA = ∠ BAC = 40 degrees (triangle ABC – isosceles);

∠ ABC = ∠ B = 180 – (40 + 40) = 180 – 80 = 100 degrees (the sum of the angles in the triangle is 180).

Answer: 100 degrees.

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