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

• A. 180 degrees
• B. 360 degrees
• C. 90 degrees
• D. 270 degrees

Answer: (A)

In a triangle, the three interior angles always add up to 180 degrees. A quick way to see this is to draw a line through the top vertex that is parallel to the base. The two angles at the base then correspond to the other two interior angles of the triangle, so all three angles line up along a straight line and sum to 180 degrees. Another way to view it is the general rule for polygon angles: for a triangle (three sides), (n−2)×180 = (3−2)×180 = 180.

The other numbers don’t fit a triangle’s interior angles: 360 degrees is the sum for shapes with four sides (or angles around a point), 90 degrees is just a single angle, and 270 degrees isn’t the sum of a triangle’s interior angles.