Perimeter of a Triangle: Formula & Examples

A triangle is three-sided shape with a height that’s perpendicular to its base. There are different types of triangle, such as scalene, isosceles, and equilateral. The formulas for finding the area and the perimeter of a triangle are the same for all its types.

Here’s an illustration of a triangle and the formulas for finding its area and perimeter.

Area and Perimeter of a triangle

Want to learn how to find the area of a triangle?
Related Reading: Area of a Triangle – Formula & Examples

How to Find the Perimeter of a Triangle

The perimeter of a triangle is the sum of all its three sides. Make sure all measurements are the same before computing the perimeter using the formula:

P = a + b + c
where a, b, and c = the three sides of the triangle

Example. Find the perimeter of a triangle with the following measurements: a = 12 in; b = 15 in; c = 12 in.

P = a + b + c = 12 in + 15 in + 12 in
P = 39 in

Therefore, the perimeter of a triangle whose sides measure 12 in, 15 in, and 12 in is 39 in.

Example #1: Find the perimeter of a triangle with the following measurements: a = 7 cm; b = 12 cm; c = 15 cm.

Example 1 Image

Solution for Example #1:

Substitute the given measurements into the equation, P = a + b + c.

P = 7 cm + 12 cm + 15 cm
P = 34 cm

Therefore, the perimeter is 34 cm.

Example #2: Find the perimeter of a triangle with the following measurements: a = 800 m; b = 0.7 km; c = 800 m.

Example 2 Image

Solution for Example #2:

Before plugging the given measurements into the perimeter equation, we must convert 0.7 km into meters so that it matches the units of the other sides. Recall that 1 km = 1000 m. Solve for b using cross multiplication.

0.7 km / 1 km = b / 1000m
700 m = b
Note: The km on the left side of the equation cancel each other out.

Therefore, b equals 700 m.

Now, substitute the given measurements into the equation, P = a + b + c.

P = 800 m + 700 m + 800 m
P = 2300 m

Therefore, the perimeter is 2,300 m.

Thank you for reading. We hope it’s effective! Always feel free to revisit this page if you ever have any questions about the perimeter of a triangle.

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