What if you were told that a triangle has sides that measure 3, 4, and 5? How could you determine which of the triangle's angles is largest? Smallest? After completing this Concept, you'll be able to use triangle theorems to solve problems like this one.

### Watch This

CK-12 Comparing Angles & Sides in Triangles

James Sousa: Proof that the Angle of a Triangle Opposite The Longest Side is the Largest Angle

### Guidance

Look at the triangle below. The sides of the triangle are given. Can you determine which angle is the largest? The largest angle will be opposite 18 because that is the longest side. Similarly, the smallest angle will be opposite 7, which is the shortest side.

This idea is actually a theorem: If one side of a triangle is longer than another side, then the angle opposite the longer side will be larger than the angle opposite the shorter side.

The converse is also true: If one angle in a triangle is larger than another angle in that triangle, then the side opposite the larger angle will be longer than the side opposite the smaller angle.

We can extend this idea into two theorems that help us compare sides and angles in **two** triangles If we have two congruent triangles

Therefore, if

Now, let’s make **SAS Inequality Theorem**.

**The SAS Inequality Theorem:** If two sides of a triangle are congruent to two sides of another triangle, but the included angle of one triangle has greater measure than the included angle of the other triangle, then the third side of the first triangle is longer than the third side of the second triangle.

If

If we know the third sides as opposed to the angles, the opposite idea is also true and is called the **SSS Inequality Theorem**.

**SSS Inequality Theorem:** If two sides of a triangle are congruent to two sides of another triangle, but the third side of the first triangle is longer than the third side of the second triangle, then the included angle of the first triangle's two congruent sides is greater in measure than the included angle of the second triangle's two congruent sides.

If

#### Example A

List the sides in order, from shortest to longest.

First, find

#### Example B

List the angles in order, from largest to smallest.

Just like with the sides, the largest angle is opposite the longest side. The longest side is

#### Example C

List the sides in order, from least to greatest.

To solve, let’s start with

For

By the SAS Inequality Theorem, we know that

CK-12 Comparing Angles & Sides in Triangles

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### Guided Practice

1. If

2. List the sides of the two triangles in order, from shortest to longest.

3. Below is isosceles triangle

**Answers:**

1.

We can use the SSS Inequality Theorem Converse to say

2. There are no congruent sides or angles. Look at each triangle separately.

Because the longest side in

3.

### Explore More

For questions 1-3, list the sides in order from shortest to longest.

For questions 4-6, list the angles from largest to smallest.

- Draw a triangle with sides 3 cm, 4 cm, and 5 cm. The angle measures are
90∘, 53∘ , and37∘ . Place the angle measures in the appropriate spots. - Draw a triangle with angle measures
56∘, 54∘ and the included side is 8 cm. What is the longest side of this triangle? - Draw a triangle with sides 6 cm, 7 cm, and 8 cm. The angle measures are
75.5∘, 58∘ , and46.5∘ . Place the angle measures in the appropriate spots. - What conclusions can you draw about
x ? - Compare
m∠1 andm∠2 . - List the sides from shortest to longest.
- Compare
m∠1 andm∠2 . What can you say aboutm∠3 andm∠4 ?

### Answers for Explore More Problems

To view the Explore More answers, open this PDF file and look for section 5.6.