Question

One of the base angles of an isosceles triangle is 70 degrees.The verticle angle is?
a.65 degrees
b.130 degrees
c.40 degrees
d.66 degrees

Answer 1

In isosceles triangle ABC

$AB = AC$ (ABC is an isosceles triangle)

$B = C$ (angle opposite to equal side in triangle are equal)

B = C = 70°

By angle sum property of triangle,

A + B + C = 180°

A + 70° + 70° = 180°

A + 140° = 180°

A = 180° - 140°

A = 40°

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Answer 2

Answer:

Let the base angles be angle A and angle B and the vertical angle be angle C

We know that the base angles of an isosceles triangle are equal. So, both the base angles = 70°

We also know that,

angle A+ angle B + angle C = 180°

[ Angle sum property]

70° + 70° + angle C = 180°

or, 140° + angle C = 180°

or, angle C = 180°-140°

or, angle C = 40°

Therefore, the vertical angle = 40°

Hence, answer is (c) 40 degrees

Hope it helps you :-)