Carpentry & Building Construction © 2004

Math Activities :

Self-Check Math Activity 19

Perimeter of a Triangle

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The perimeter of a triangle or any other polygon is the sum of the lengths of its sides. Carpenters may have to find the perimeter of a triangle when figuring the amount of trim or estimating how much lumber is needed.

For some types of triangles, you can find the perimeter even when the length of one side is unknown. For example, an equilateral triangle has three equal sides. If you know that two of the sides are 10'', then the third is 10'' as well. The perimeter of this triangle is 10'' + 10'' + 10'' = 30''.

Terms, Symbols, & Abbreviations
perimeter – sum of the sides of a polygon
triangle – a polygon with three sides and three angles
isosceles triangle – a triangle with two equal sides and two equal angles
equilateral triangle – a triangle with three equal sides and three equal angles
scalene triangle – a triangle with three different side lengths and three different angles

Practice Exercise

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Find the perimeter of this triangle.
Step 1 Add the sides of the triangle.
  16 + 18 + 22 = 56
The perimeter of the triangle is 56.

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Find the perimeter of an isosceles triangle when the base is 8 and one of the sides is 5.
Step 1 An isosceles triangle has two equal sides; so if one side is 5, then the other side is 5 also.
  8 + 5 + 5 = 18
The perimeter of the isosceles triangle is 18.

Problem Exercises

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Refer to the triangles in the following figure for Problems 1 through 4.

1.
Find the perimeter of the right triangle.
A)16
B)18
C)24
D)200
2.
Use what you know about the isosceles triangle to find its perimeter.
A)28.97
B)40.97
C)45.94
D)50.91
3.
Find the perimeter of the equilateral triangle.
A)11
B)22
C)33
D)44
4.
What is the perimeter of the scalene triangle?
A)27
B)38
C)45
D)54

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5.
Find the perimeter of the triangle.
A)25.5
B)28.5
C)30
D)42
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