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

CK-12 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*}, so they would all lie on the same line. The lengths 4, 5, 10 also cannot make a triangle because \begin{align*}4 + 5 = 9\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*} and \begin{align*}12>11\end{align*} so **yes** these lengths make a triangle.

#### 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*}, can either be the shortest side or the longest side. For example \begin{align*}s\end{align*} could be 5 because \begin{align*}6 + 5 > 10\end{align*}. It could also be 15 because \begin{align*}6 +10 > 15\end{align*}. Therefore, the range of values for \begin{align*}s\end{align*} is \begin{align*}4 < s < 16\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*}. For the same reason, \begin{align*}s\end{align*} cannot be greater than 16, but it could 15.9, \begin{align*}10 + 6 > 15.9\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*} is the length of a leg:

\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.

CK-12 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.

### 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*}. Yes this is a triangle because \begin{align*}7.6 > 7.5\end{align*}.

2. \begin{align*}4 + 4 = 8\end{align*}. No this is not a triangle because two lengths ** cannot equal** the third.

3. \begin{align*}6 + 7 > 8\end{align*}. Yes this is a triangle because \begin{align*}13 > 8\end{align*}.

### Explore More

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*} and \begin{align*}2x\end{align*}
- The legs of an isosceles triangle have a length of 12 each. What can you say about the length of the base?