Geometry and TrigonometryMediumBase angle of an isosceles triangle from its apex
Geometry and Trigonometry practice question
In triangle ABC, AB = BC. If angle B measures 36°, what is the measure, in degrees, of angle A?
Answer: 72
Since AB = BC, the angles opposite those sides — angles C and A — are equal. They share 180 − 36 = 144°, so each is 72°.
Common wrong answers
- If you answered 36
- 36 is angle B, the apex; the base angles are different.
- If you answered 54
- 54 is 90 − 36, as if angle A and angle B were complementary.
- If you answered 144
- 144 is the two base angles TOGETHER; each is half of that.
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