Question

The measure of ∠DAB equals 110º. The measure of ∠ACB equals 20º.

What is m∠ABC?



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°
Triangle A B C with ray A D extended from side A C.

Answer 1

The exterior-angle theorem gives ∠DAB = ∠ABC + ∠ACB. So 110° = ∠ABC + 20°, hence ∠ABC = 110° − 20° = 90°.