Q10 of 23 Page 87

In the given figure at right, side BC of ΔABC is produced to D. Find ∠A and ∠C.

Given ∠B = 40° and ∠ACD = 120°


∵ ∠ACD is an external angle


∴ ∠ACD = ∠A + ∠B


⇒ ∠A + 40° = 120°


⇒ ∠A = 80°


We know that sum of the angles of a triangle is 180°.


∠A + ∠B + ∠C = 180°


⇒ 80° + + 40° + ∠C = 180°


⇒ ∠C = 180° - 120°


⇒ ∠C = 60°


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