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 interior angles of a triangle?

Back

The sum of the interior angles of a triangle is always 180°.

Front

If two angles of a triangle are equal, what can be said about the triangle?

Back

If two angles of a triangle are equal, the triangle is isosceles.

Front

In an isosceles triangle, if the vertex angle is 40°, what are the measures of the base angles?

Back

The base angles will each measure 70° because the sum of the angles must equal 180°.

Front

What is the relationship between the sides and angles in an equilateral triangle?

Back

In an equilateral triangle, all sides are equal, and all angles are equal to 60°.

Front

How do you find the measure of an unknown angle in a triangle?

Back

To find the measure of an unknown angle in a triangle, subtract the sum of the known angles from 180°.

Front

If one angle of an isosceles triangle is 80°, what are the measures of the other two angles?

Back

The other two angles will each measure 50° because the sum of the angles must equal 180°.

Front

What is the formula for the angles in an isosceles triangle?

Back

Front

In triangle ABC, if AB = AC, what can be concluded about angles B and C?

Back

Angles B and C are equal because the sides opposite to them are equal.