The perimeter of a triangle is the distance around the outside of the triangle. You can find the perimeter of a triangle by adding up all of the side lengths of the triangle. If you are missing a side length, you can use the Pythagorean Theorem, Trig Ratios, Law of Sines, or Law of Cosines to find the missing side length.
How to Find the Perimeter of a Triangle
- Find all three side lengths of the triangle.
- Add the side lengths together.
Perimeter of an Equilateral Triangle
All three sides of an equilateral triangle are equal in length so you really only need to know the length of one of the sides of the triangle to find the perimeter of an equilateral triangle
Perimeter of a Right Triangle
When you are trying to find the perimeter of a right triangle, you can use the Pythagorean Theorem or Trig Ratios to find missing side lengths, if needed.
Perimeter of an Acute or Obtuse Triangle
If you have missing side lengths on an acute or obtuse triangle, you can use the Law of Sines or the Law of Cosines to find the missing side lengths.
Semiperimeter of a Triangle
The semiperimeter of a triangle is half of the perimeter of the triangle. It is used in Heron’s Formula to find the area of a triangle and it can also be used to find the circumradius of a triangle.