Geometry and TrigonometryEasy–MediumBase angles of an isosceles triangle
Geometry and Trigonometry practice question
In an isosceles triangle, the angle between the two equal sides measures 40°. What is the measure of each of the other two angles?
- A. 70°Correct
- B. 140°
- C. 40°
- D. 50°
Answer: A. 70°
In an isosceles triangle the two angles opposite the equal sides are equal. They share what's left of 180° after the 40°: (180° − 40°) / 2 = 70° each.
Why the other answers are wrong
- B. 140°
- 140° is the total of the two equal angles, 180° − 40°. Each one is half of that.
- C. 40°
- The 40° angle is the one BETWEEN the equal sides; the other two are equal to each other, not to it.
- D. 50°
- 50° is 90° − 40°, which would only apply in a right triangle. Nothing says this triangle has a right angle; use 180°.
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