The perimeter of a triangle is the total distance around its three outer sides. If a triangle has side lengths equal to D, E and F, then its perimeter is the addition of D, E and F.
When three sides of a triangle are all equal, the shape is called a equilateral triangle. The perimeter of an equilateral triangle, which has each side length equal to A, has a perimeter equal to (A + A + A), or 3A.
For any polygonal shape, the perimeter is given as the distance around its outer sides. A square, which has four sides with lengths each equal to Y, has a perimeter equal to 4 x Y. Similarly, a rectangle, which has two sides equal to R and the other two equal to S, has the perimeter of (2 x R + 2 x S).