The perimeter of the triangle is 39 cm. Its two sides are 12 cm and 15 cm. Find a third party.

The perimeter of a triangle (P) is equal to the sum of the lengths of all its sides:

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

Knowing the perimeter of the triangle and the lengths of the two sides, the length of the third side can be found by the formula:

c = P – a – b = P – (a + b).

We calculate the third side of the triangle, in which P = 39 cm, a = 12 cm, b = 15 cm:

c = 39 – (12 + 15) = 39 – 27 = 12 cm.

Answer: The length of the third side of the triangle is 12 cm.