Isosceles & Equilateral Triangle Angle Theorems

Front

What is an isosceles triangle?

Back

An isosceles triangle is a triangle that has at least two sides of equal length. The angles opposite these sides are also equal.

Front

What is an equilateral triangle?

Back

An equilateral triangle is a triangle in which all three sides are of equal length and all three angles are equal, each measuring 60°.

Front

What is the sum of the angles in any triangle?

Back

The sum of the angles in any triangle is always 180°.

Front

What is the vertex angle in an isosceles triangle?

Back

The vertex angle in an isosceles triangle is the angle formed by the two equal sides.

Front

How do you find the base angles of an isosceles triangle?

Back

To find the base angles of an isosceles triangle, subtract the vertex angle from 180° and then divide by 2.

Front

If one angle of an isosceles triangle is 40°, what are the base angles?

Back

The base angles are each 70° because 180° - 40° = 140°, and 140° / 2 = 70°.

Front

What is the Isosceles Triangle Theorem?

Back

The Isosceles Triangle Theorem states that in an isosceles triangle, the angles opposite the equal sides are equal.

Front

What is the Equilateral Triangle Theorem?

Back

The Equilateral Triangle Theorem states that all angles in an equilateral triangle are equal to 60°.

Front

How do you solve for an unknown angle in an isosceles triangle?

Back

Use the property that the sum of the angles in a triangle is 180°. If you know the vertex angle, subtract it from 180° and divide by 2 to find the base angles.

Front

If the vertex angle of an isosceles triangle is 50°, what are the base angles?

Back

The base angles are each 65° because 180° - 50° = 130°, and 130° / 2 = 65°.