How to Find the Perimeter of a Triangle

When Knowing the Lengths of All Three Sides

1. 1
   Know the formula for finding the perimeter. The formula is: A + B + C = X where A, B, and C represent the lengths of the sides and X represents the perimeter.
   
   

   

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2. 2
   Find the value of all three sides. In this example: A = 5, B = 5, C = 5. Here, you're working an equilateral triangle, because all three sides of the triangle have the exact same length.
   
   

   

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3. 3
   Add up the lengths all three sides. In this example: 5 + 5 + 5 = 15. The area of the triangle is 15 units. Since it is an equilateral triangle, you can also use the formula, P = 3S, where P represents the perimeter and S represents each side. You can just multiply the length of one side by 3 instead of adding up all three sides, since you'll be effectively doing the same thing.
   
   

   

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4. 4
   Always remember to include the units in your final answer. In this example, the sides are measured in centimeters, so your final answer should use centimeters as well. The final answer is: X = 15 cm.
   
   

   

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When Missing the Length of a Right Triangle's Hypotenuse

1. 1
   Write down the Pythagorean Theorem. The formula is: A² + B² = C². In this formula, A and B represent the legs of the triangle and C represents its hypotenuse, or the longest side of the triangle, which is across from the right angle.
   
   

   

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2. 2
   Use the Pythagorean Theorem to find the length of the hypotenuse. You know that A and B are 6 and 8. Now, simply plug those numbers in to the formula. Here's how you do it:

   • A² + B² = C²
   • 6² + 8² = C²
   • 36 + 64 = C²
   • 100 = C²
   • √(100) = √C²
   • 10 = C
3. 3
   Add up the length of the three sides. Now that you know that sides A, B, and C are 6 cm, 8 cm, and 10 cm long, just add up the length of the three sides to get the perimeter. 6 + 8 + 10 = 24 cm.
   
   

   

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