Question

if an isosceles triangle has a base angle measuring 40°, then what measure is the vertex angle? 120° 100° 40° 80°

Explanation:

Step1: Recall the property of isosceles triangle

In an isosceles triangle, the two base angles are equal.

Step2: Use the angle - sum property of a triangle

The sum of angles in a triangle is \(180^{\circ}\). Let the vertex angle be \(x\). Given base angle \(= 40^{\circ}\), and since there are two base angles in an isosceles triangle, we have the equation \(x + 40^{\circ}+40^{\circ}=180^{\circ}\).

Step3: Solve the equation for \(x\)

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Answer:

\(100^{\circ}\) (the blue - colored option)