What is Isosceles Triangle?

In short

An isosceles triangle has two equal sides, and the angles opposite those sides are also equal.

In simple words

The theorem works both ways. If two sides are equal, the angles opposite them are equal. If two angles are equal, the sides opposite them are equal. So in triangle $STU$ with $\angle S = \angle U$ , the sides opposite, $TU$ and $ST$ , are equal. The perpendicular from the apex to the base bisects both the base and the angle at the apex.

Example

In $\triangle STU$ , $\angle S = \angle U$ and $TU = 20$ . The side opposite $\angle S$ is $TU$ , and the side opposite $\angle U$ is $ST$ , so $ST = TU = 20$ .

Also searched as: congruent triangles, angles opposite equal sides.