A triangle is called isosceles if its two sides are equal. These equal sides are called sides, and the third side is called the base of the triangle.
The properties of the angles at the base of an isosceles triangle are expressed as theorems.
Theorem: In an isosceles triangle, the angles at the base are equal. And the converse theorem: if two angles in a triangle are equal, then it is isosceles.
The angles at the base of an isosceles triangle will always be acute, i.e. less than 90 degrees.
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