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In a triangle the sides are 4 cm, 5 cm and 4 cm. One of the base angle is 80°. Find the measure of the apex angel.

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In a triangle the sides are 4 cm, 5 cm and 4 cm. One of the base angle is 80°. Find the measure of the apex angel.

(a) 30°

(b) 40°

(c) 20°

(d) 10°

I had been asked this question during an internship interview.

This question is from Quadrilaterals topic in chapter Quadrilaterals of Mathematics – Class 8

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The correct choice is (c) 20°

For explanation: One can get the hint that the given triangle is isosceles triangle by observing the length of all the sides. As in an isosceles triangle two sides are of equal length and also the base angles are equal in measure. Let the apex angle for denoted by the variable x.

Therefore 80+80+x=180

Therefore 160+x=180

Therefore x=20.

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