Which property defines an equilateral triangle?

Which property defines an equilateral triangle?

• A. It has a right angle.
• B. All three sides are equal.
• C. All three angles are 120 degrees.
• D. It has exactly two sides equal.

Answer: (B)

An equilateral triangle is defined by having all three sides the same length. Because the sides are equal, the angles opposite those sides are also equal, and since the interior angles of any triangle add up to 180 degrees, each angle must be 60 degrees. The other options describe different shapes: a right angle would make it a right triangle, three angles of 120 degrees would exceed the 180-degree total, and having exactly two equal sides describes an isosceles triangle. So the defining property is that all three sides are equal.

An equilateral triangle is defined by having all three sides the same length. Because the sides are equal, the angles opposite those sides are also equal, and since the interior angles of any triangle add up to 180 degrees, each angle must be 60 degrees. The other options describe different shapes: a right angle would make it a right triangle, three angles of 120 degrees would exceed the 180-degree total, and having exactly two equal sides describes an isosceles triangle. So the defining property is that all three sides are equal.