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What is the base angle of an isosceles triangle, if the vertex angle is $40 °$ ?

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70 °

40 °

80 °

60 °

answer is A.

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Detailed Solution

We have been given that,
One of the angles of a triangle =

40 ° .

Let’s assume that the base angle be b

°

.
We know that the angles sum of a triangle is

180 ° .

Then we get,

b ° + b ° + 40 ° = 180 ° ⇒ 2 b ° + 40 ° = 180 °

⇒ 2 b ° = 180 ° − 40 ° ⇒ 2 b ° = 140 ° ⇒ b ° = 70 °

Hence, in the isosceles triangle, the base angles is

70 ° .

Thus, option (1) is correct.

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