Geometry and TrigonometryVery hardExterior angle at the vertex of an isosceles triangle
Geometry and Trigonometry practice question
In triangle ABC, AB = AC and angle B measures 70°. Side BA is extended past A to point D. What is the measure, in degrees, of angle CAD?
Answer: 140
Base angles B and C are both 70°, so angle A = 180 − 140 = 40°. Angle CAD is the exterior angle at A, supplementary to it: 180 − 40 = 140°. (Equivalently, it equals B + C = 140.)
Common wrong answers
- If you answered 40
- 40 is the INTERIOR angle at A; the exterior angle makes a straight line with it.
- If you answered 70
- 70 is a base angle.
- If you answered 110
- 110 is 180 − 70, the exterior angle at B or C, not at A.
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