The sum of the first and second edges of the triangle is 18.5 cm, the sum of the second and third edges is 15.8 cm.

The sum of the first and second edges of the triangle is 18.5 cm, the sum of the second and third edges is 15.8 cm. The sum of the first and third edges is 17.7 cm. Find the perimeter of the triangle.

1. Let the first edge of the triangle be x, then the second – y, and the third – z.
2. We compose a triple system of equations:
{x + y = 18.5 (1);
{y + z = 15.8 (2);
{x + z = 17.7 (3);
3. We sum up these equations and get:
x + y + y + z + x + z = 18.5 + 15.8 + 17.7;
2 * x + 2 * y + 2 * z = 52;
2 * (x + y + z) = 52;
4. Divide both parts by 2 and find the perimeter of the triangle:
P = x + y + z = 26;
5. Answer: P = 26.