In a ∆ABC, BC is extended upto D. ∠ACD = 120°,

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We draw a figure triangle ABC whose BC is extended upto D point ,

Exterior ∠ACD = Interior (∠BAC + ∠ABC)

⇒ 120° = ∠A +	∠A
⇒ 120° = ∠A +	2

⇒ 240 = 2∠A + ∠A
⇒ 3∠A = 240°

⇒ ∠A =	240°	= 80°
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