The perimeter is a unit of measurement that calculates the distance around a closed shape, such as a triangle. To find the perimeter of a triangle -- assuming you know the length of the triangle's three sides -- you simply add the three sides together.

## Perimeter of a Triangle

**To find the perimeter of a triangle** -- with sides a, b, and c -- you add the lengths of the three sides. In this example, the lengths of the sides are 4 inches, 3 inches and 4 inches, respectively. You add 4 inches + 3 inches + 4 inches. This results in a perimeter of 11 inches.

## Equilateral and Isosceles Triangles

**If you have an *equilateral triangle** *where all sides are two inches in length, you add 2 inches + 2 inches + 2 inches. This results in a perimeter of 6 inches. If you have an* isosceles triangle,* two sides will have the same length, but the third will have a different length. In this example, two sides are two inches each and one side is three inches. You add 2 inches + 2 inches + 3 inches. This results in a perimeter of 7 inches.

## Obtuse, Acute and Right Triangles

**Whether you have an obtuse, acute or a right triangle** -- the formula is the same as for any other triangle. An *obtuse* triangle has one internal angle *greater* than 90 degrees. An *acute* triangle has one internal angle *less* than 90 degrees. A *right* triangle has one internal angle *exactly* 90 degrees. To find the perimeter of any of these triangles, simply add the lengths of the three sides of the respective triangle.

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About the Author

Bonnie Crowe is a mother of two teenagers; a teacher and author of children's books, curriculum and articles on English grammar, literature, technology, art, parenting and career guides for high schoolers. She's a former director of AOL Parenting, a member of SCBWI, and a graduate from the University of California,Berkeley.

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