Find the side of an equilateral triangle if it is less than 42 cm from the perimeter of the triangle.

1. Let’s denote the side of an equilateral triangle by x.

2. To calculate the side we will use the formula: the perimeter of a triangle is equal to the sum of its three sides.

But according to the condition of the problem, it is known that all sides are equal to each other, which means that the perimeter = x + x + x.

3. We know that one side is 42 cm less than the perimeter, that is, less than the sum of the three sides of the triangle – we will compose and solve the equation:

x = x + x + x – 42 cm;

3 x = 42 cm;

x = 42 cm: 3 = 21 cm.

Answer: The side of an equilateral triangle is 21 centimeters.