What is the sum of the interior angles in a triangle?

• A. 360
• B. 90
• C. 180
• D. 270

Answer: (C)

The interior angles of a triangle always add up to 180 degrees. A clear way to see this is to draw a line through the top vertex parallel to the base. The angles at the bottom of the triangle correspond to two angles formed with that parallel line, so those three angles line up along a straight line. A straight line measures 180 degrees, so the three interior angles must sum to 180. For any triangle, whether it’s acute, obtuse, or right-angled, the total of its interior angles is 180 degrees.