The perimeter of the triangle is 20cm. The lengths of its two sides are 7cm and 8cm. Find the length of the third side.

From the condition, we know that the perimeter of the triangle is 20 cm. The lengths of its two sides are 7 cm and 8 cm, respectively. You need to find the length of the third side.

Let’s remember what a perimeter is. Perimeter in mathematics is the sum of the lengths of all sides of a geometric figure.

So we can write the formula for finding the perimeter of a triangle as follows.

P = a + b + c, where a, b, c are the lengths of the sides of the triangle.

Substitute the known values and solve the equation:

7 + 8 + c = 20;

c = 20 – 7 – 8;

c = 5.

Answer: 5 cm is the length of the third side of the triangle.