Sum of interior angles in any triangle is

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

Answer: (A)

Think about a triangle on a flat surface. If you extend one side to form an exterior angle at a vertex, that exterior angle equals the sum of the other two interior angles. The interior angle at that vertex and the exterior angle lie on a straight line, so together they measure 180 degrees. Since the exterior angle equals the sum of the other two interior angles, adding all three interior angles gives 180 degrees. This holds for any triangle in Euclidean geometry, so the total is 180 degrees.