A triangle has one angle of 70 degrees and the other two are equal and smaller. What is this triangle called?

• A. Right Triangle
• B. Obtuse Triangle
• C. Acute Triangle
• D. Equilateral Triangle

Answer: (C)

This question tests classification of a triangle by angles. If one angle is 70 degrees and the other two are equal, those two angles must each be (180 − 70) / 2 = 55 degrees. So the angles are 70°, 55°, and 55°. Since all three angles are less than 90 degrees, the triangle is an acute triangle. It isn’t a right triangle (no 90° angle) and it isn’t an obtuse triangle (no angle greater than 90°). It also isn’t equilateral because the angles aren’t all equal (an equilateral triangle would have all angles 60°).