In geometry, an isosceles triangle is a triangle that has two sides of equal length. Sometimes it is specified as having exactly two sides of equal length, and sometimes as having at least two sides of equal length, the latter version thus including the equilateral triangle as a special case. The two equal sides of an isosceles triangle are known as ‘legs’ whereas the third or unequal side is known as the ‘base’.
Angles opposite to equal sides in an isosceles triangle are always of equal measure.
By the Steiner - Lehmus theorem, every triangle with two angle bisectors of equal length is isosceles. The median from the vertex to the base is perpendicular to the base. The median from the vertex to the base divides the triangle into two congruent right-angled triangles.
Chapter 10 of class 9 gives the students an overview of the different properties and problems related to the Isosceles Triangle.