Question

What is M A.

42°


B.
50°


C.
58°


D.
100°

Answer 1

C

Explanation:

The external angle of a triangle is equal to the sum of the 2 opposite interior angles.

∠ACD is an exterior angle of the triangle and

∠A and ∠B are the 2 opposite interior angles, hence

∠A + ∠B = 138, that is

∠A + 80 = 138 ( subtract 80 from both sides )

∠A = 58° → C

Answer 2

C. 58°

Explanation:

m∠ACB=(180-138)°

m∠B + m∠A +∠mACB =180

80+ m∠A + (180 -138) =180

80 + m∠A - 138 = 0

m∠A = 58