Perimeter of a triangle

The perimeter of a triangle is the sum of the lengths of all three of its sides. Let the lengths of the three sides be a, b and c. Then the perimeter of the triangle, which we denote P, is

$$P=a+b+c$$

Alternatively, for a triangle ABC we have

$$P=|AB|+|BC|+|CA|$$

where |AB| is the length of side AB. The triangle below has side lengths a = 3, b = 5 and c = 4, so its perimeter is

$$P=3+5+4=12$$