The perimeter of the triangle is 62 cm, the length of one side is 18 cm, the other is 21 cm. Find the length of the third side.

From the condition it is known that the perimeter of a triangle is 62 cm, and the length of one side is 18 cm, the other – 21 cm. In order to find the length of the third side, recall the definition of the perimeter of a triangle.
So, the perimeter is the sum of the lengths of all sides of a geometric figure.
For a triangle, the formula for finding the perimeter looks like this:
P = a + b + c;
Let’s denote the third side by the variable x.
Substitute and calculate:
18 + 21 + x = 62;
39 + x = 62;
x = 62 – 39;
x = 23 cm length of the third side of the triangle.