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

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

Answer: (B)

The sum of the interior angles of a triangle is always 180°. This fact arises from the fundamental properties of triangles in Euclidean geometry.

To understand why this is the case, consider that a triangle can be divided into two right triangles by drawing a line from one vertex perpendicular to the opposite side. Each right triangle has one angle that measures 90°, and since the angles of the triangle must also combine to form a straight line at the base, the remaining angles (the two acute angles of the triangle) must together add up to 90°. Therefore, when these angles are summed, the total is 90° + 90° (from the right angle triangles) + the two acute angles, equaling 180°.

This inherent property holds true regardless of the type (scalene, isosceles, equilateral) of triangle being considered. Hence, the correct answer is that the sum of the interior angles of a triangle is 180°.