ISEE Lower Level Math : How to find the perimeter of a triangle
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Example Questions
Example Question #332 : Geometry
The triangle below has a perimeter of 
What is the length of the third side?
Remember, the perimeter of a triangle is the sum of its three sides. First, sum up the two side lengths that we are given in the question:
To find the third side, subtract the sides we know from the total perimeter:
Example Question #333 : Geometry
An equilateral triangle has a perimeter of 
An equilateral triangle has three congruent sides, i.e. the sides are all the same length.
The perimeter of an equilateral triangle is given by 
Thus, 
Example Question #1 : How To Find The Perimeter Of A Triangle
Use the triangle to answer the question.
What is the perimeter of the triangle?
To find the perimeter, add all the sides together.
Example Question #2 : How To Find The Perimeter Of A Triangle
An equilateral triangle has a side length equal to 
What is the perimeter of the triangle?
Because it is an equilateral triangle, all sides have the same length 
The perimeter is the sum of the length of the sides.
Example Question #3 : How To Find The Perimeter Of A Triangle
Figure not drawn to scale
The perimeter of isosceles triangle 
If 
The perimeter of the triangle is equal to the sum of the lengths of all the sides.
Because the triangle is an isosceles triangle, the triangle has two equal sides, which we know are 
Since 
We now solve for 
Example Question #4 : How To Find The Perimeter Of A Triangle
What is the name of the longest side of a right triangle?
Hypotenuse
Obtuse
Base
Equilateral
Length
Hypotenuse
The hypotenuse is the longest side of a triangle with a right angle.
All the other terms refer to something else. Obtuse refers to an angle over 90 degrees. Length is too general of a term, and refers to the measurement of any given side. Equilateral refers to a triangle in which all angles and side are of equal measurement. Base refers to the side that is perpendicular to the measure of the height, meaning that it will be one of the shorter sides.
Example Question #6 : How To Find The Perimeter Of A Triangle
In an isosceles triangle, two side are 4 feet long. Which of the following is NOT a possible length of the third side?
5 feet
8 feet
1 foot
2.5 feet
7 feet
8 feet
For triangles, it is impossible for a third side to be equal to or greater than the sum of the two other sides.
Given that the sum of the two sides of this triangle is 8, it is impossible for the third side to be equal to 8 feet.
Example Question #5 : How To Find The Perimeter Of A Triangle
In an equilateral triangle, the perimeter is 42. What is the length of 1 side?
In an equilateral triangle, all 3 sides are equal to one another. Therefore, if the perimeter is 42, the length of 1 side will be equal to 42 divided by 3. This results in 14, which is therefore the correct answer.
Example Question #1 : How To Find The Perimeter Of A Triangle
If the side of an equilateral triangle is equal to 
The three sides must sum to 18:
Combine like terms on the left:
Divide both sides by 9:
Example Question #6 : How To Find The Perimeter Of A Triangle
If an equilateral triangle has sidelengths of 3 inches, what is the perimeter?
An equilateral triangle is a speical type of triangle that has three congruent sides; in other words, all sides are the same length.
Therefore the perimeter for an equilateral triangle can be written as 
Plug in the sidelength given in the question to find the perimeter:
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