An isosceles triangle is a type of triangle that has two sides of equal length. The angles opposite to the two equal sides are also equal in measure. Isosceles triangles have been studied in mathematics since ancient times. They are classified into three different types based on their angles:

• Isosceles acute triangle: A triangle in which all three angles are less than 90°, and at least two of its angles are equal in measurement.
• Isosceles right triangle: A right triangle with two legs (and their corresponding angles) of equal measure.
• Isosceles obtuse triangle: A triangle in which one of the three angles is obtuse (lies between 90° and 180°), and the other two acute angles are equal in measurement.

In an isosceles triangle, the two equal sides are called legs, and the third side is called the base. The area of an isosceles triangle is given by the formula: Area (A) = ½ × base (b) × height (h) . The perimeter of an isosceles triangle is given by the formula: Perimeter = 2 × leg length + base length.