FTCE Professional Education Practice Exam

How is an isosceles triangle defined?

1. It has all sides equal

2. It has two sides the same length

3. It has three sides of different lengths

4. It has no sides of equal length

The correct answer is: It has two sides the same length

An isosceles triangle is defined by having at least two sides that are the same length. This characteristic is fundamental to isosceles triangles, setting them apart from other triangle types. Additionally, the two equal sides are often referred to as the legs, while the third side, which may vary in length, is known as the base. The equal sides lead to the angles opposite them also being equal, which is a critical geometric property of isosceles triangles. In contrast, a triangle that has all sides equal would be an equilateral triangle. A triangle with three sides of different lengths is categorized as a scalene triangle, and one that has no sides of equal length also fits into this scalene classification. Thus, the definition of an isosceles triangle is specifically focused on the presence of at least two equal-length sides, making the choice stating that it has two sides the same length the accurate description.