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Problem 176

Geometry Difficulty 1.3 Multiple choice CEMC Gauss (Grade 7) · Canada · 2018

The measure of one angle of an isosceles triangle is 5050^\circ. The measures of the other angles in this triangle could be

Pick one

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Official solution

The sum of the three angles in any triangle is 180180^{\circ}.

If one of the angles in an isosceles triangle measures 5050^{\circ}, then the sum of the measures of the two unknown angles in the triangle is 18050=130180^{\circ}-50^{\circ}=130^{\circ}.

Since the triangle is isosceles, then two of the angles in the triangle have equal measure.

If the two unknown angles are equal in measure, then they each measure 130÷2=65130^{\circ}\div2=65^{\circ}.

However, 6565^{\circ} and 6565^{\circ} is not one of the given answers.

If the measure of one of the unknown angles is equal to the measure of the given angle, 5050^{\circ}, then the third angle in the triangle measures 1805050=80180^{\circ}-50^{\circ}-50^{\circ}=80^{\circ}.

Therefore, the measures of the other angles in this triangle could be 5050^{\circ} and 8080^{\circ}.

Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.