The side of an isosceles triangle is 7 cm. The perimeter of the triangle is 20 cm. Find its base.

Since this triangle ABC with base AC is isosceles, the sides at the base are equal. That is, AB = BC. According to the condition of the problem, the lateral side of an isosceles triangle is 7 cm, therefore AB = BC = 7 cm. The perimeter of a triangle is equal to the sum of the lengths of all its sides: P = AB + BC + AC. Hence, the value of the length of the AC side is: AC = P – AB – BC = 20 cm – 7 cm – 7 cm = 20 cm – 14 cm = 6 cm.
Answer: the sides of the triangle are AB = 7 cm, BC = 7 cm and AC = 6 cm.