5.7: Triangle Inequality Theorem
What if you had to determine whether the three lengths 5, 7 and 10 make a triangle? After completing this Concept, you'll be able to use the Triangle Inequality Theorem to determine if any three side lengths make a triangle.
Watch This
CK12 Foundation: Chapter5TriangleInequalityTheoremA
James Sousa: Triangle Inequality Theorem
Guidance
Can any three lengths make a triangle? The answer is no. There are limits on what the lengths can be. For example, the lengths 1, 2, 3 cannot make a triangle because \begin{align*}1 + 2 = 3\end{align*}
The arc marks show that the two sides would never meet to form a triangle. The Triangle Inequality Theorem states that the sum of the lengths of any two sides of a triangle must be greater than the length of the third.
Example A
Do the lengths 4, 11, 8 make a triangle?
To solve this problem, check to make sure that the smaller two numbers add up to be greater than the biggest number. \begin{align*}4+8=12\end{align*}
Example B
Find the length of the third side of a triangle if the other two sides are 10 and 6.
The Triangle Inequality Theorem can also help you find the range of the third side. The two given sides are 6 and 10, so the third side, \begin{align*}s\end{align*}
Notice the range is no less than 4, and not equal to 4. The third side could be 4.1 because \begin{align*}4.1 + 6 > 10\end{align*}
Example C
The base of an isosceles triangle has length 24. What can you say about the length of each leg?
To solve this problem, remember that an isosceles triangle has two congruent sides (the legs). We have to make sure that the sum of the lengths of the legs is greater than 24. In other words, if \begin{align*}x\end{align*}
\begin{align*}x+x&>24\\ 2x &>24\\ x&>12\end{align*}
Each leg must have a length greater than 12.
Watch this video for help with the Examples above.
CK12 Foundation: Chapter5TriangleInequalityTheoremB
Concept Problem Revisited
The three lengths 5, 7, and 10 do make a triangle. The sum of the lengths of any two sides is greater than the length of the third.
Vocabulary
An isosceles triangle is a triangle with two congruent sides. The congruent sides are called the legs and the third side is called the base. The Triangle Inequality Theorem states that to make a triangle, two sides must add up to be greater than the third side.
Guided Practice
Do the lengths below make a triangle?
1. 4.1, 3.5, 7.5
2. 4, 4, 8
3. 6, 7, 8
Answers:
Use the Triangle Inequality Theorem. Test to see if the smaller two numbers add up to be greater than the largest number.
1. \begin{align*}4.1 + 3.5 > 7.5\end{align*}
2. \begin{align*}4 + 4 = 8\end{align*}
3. \begin{align*}6 + 7 > 8\end{align*}
Practice
Determine if the sets of lengths below can make a triangle. If not, state why.
 6, 6, 13
 1, 2, 3
 7, 8, 10
 5, 4, 3
 23, 56, 85
 30, 40, 50
 7, 8, 14
 7, 8, 15
 7, 8, 14.99
If two lengths of the sides of a triangle are given, determine the range of the length of the third side.
 8 and 9
 4 and 15
 20 and 32
 2 and 5
 10 and 8

\begin{align*}x\end{align*}
x and \begin{align*}2x\end{align*}2x  The legs of an isosceles triangle have a length of 12 each. What can you say about the length of the base?
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Here you'll learn the Triangle Inequality Theorem, which will help you to determine whether three side lengths will create a triangle or not.