The perimeter of an isosceles triangle is 68 cm, the lateral side is 20 cm. Find the foundation.

1. An isosceles triangle is a triangle in which the angles at the base and sides (a and b) are equal, that is, a = b will be correct.

2. The perimeter of a triangle is the sum of the lengths of its sides.

P = a + b + c, where P is the perimeter, a and b are the sides, c is the base of the triangle.

Since a = b, it will be true P = 2 * a + c.

3. By the condition of the problem, we make up the following expression 68 = 2 * 20 + s.

4. We make calculations c = 68 – 2 * 20, c = 28 (cm).

Answer: the length of the base of the triangle is 28 cm.