What Is an Isosceles Triangle?
An isosceles triangle has two equal angles (and two equal sides). The angles can vary, as long as two are equal. For example, a triangle with two 70º angles and one 40º angle would be an isosceles triangle.
In addition to three sides, every triangle also has three angles. Those three angles always add up to 180º. Mathematicians have given triangles special names based upon how many of those angles are equal.
For example, an equilateral triangle has three equal angles (and three equal sides). Each angle in an equilateral triangle equals 60º, since three 60º angles would add up to 180º.
If you’ve been counting carefully, you know there’s one more option. If a triangle doesn’t have two or three equal angles, it could have zero equal angles. These triangles, which also don’t have any equal sides, are known as scalene triangles.
Geometry experts classified triangles based upon the type of angles inside the triangle too. For example, a triangle with a right angle, which is an angle equal to 90º, is known as a right triangle.
Some triangles, however, have three angles that are all less than 90º. Such a triangle is called an acute triangle. A triangle can also have an angle more than 90º. Those triangles are called obtuse triangles.