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In a triangle ABC, AB = AC,∠BAC = 40° Then the external angle at B is :
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- 90°
- 70°
- 110°
- 80°
- 90°
Correct Option: C
On the basis of given in question , we draw a figure of an isosceles triangle ABC ,
∠ABC = ∠ACB [∵ AB = AC]
∠BAC = 40°
We know that , ∠BAC + ∠ABC + ∠ACB = 180°
⇒ ∠ABC + ∠ACB = 140°
⇒ 2∠ABC = 140° { ∴ ∠ABC = ∠ACB }
⇒ ∠ABC = 70°
∴ ∠ABD = 180° – 70° = 110°