What if you were given two triangles and provided with only two of their side lengths and the measure of the angle between those two sides? How could you determine if the two triangles were congruent? After completing this Concept, you'll be able to use the Side-Angle-Side (SAS) shortcut to prove triangles are congruent.
Watch This
CK-12 Foundation: Chapter4SASTriangleCongruenceA
Watch the portions of the following two videos that deal with SAS triangle congruence.
James Sousa: Introduction to Congruent Triangles
James Sousa: Determining If Two Triangles are Congruent
James Sousa: Example 1: Prove Two Triangles are Congruent
Guidance
An included angle is when an angle is between two given sides of a triangle (or polygon). In the picture below, the markings indicate that and are the given sides, so would be the included angle.
Consider the question: If I have two sides of length 2 in and 5 in and the angle between them is , can I construct only one triangle?
Investigation: Constructing a Triangle Given Two Sides and Included Angle
Tools Needed: protractor, pencil, ruler, and paper
- Draw the longest side (5 in) horizontally, halfway down the page. The drawings in this investigation are to scale.
- At the left endpoint of your line segment, use the protractor to measure a angle. Mark this measurement.
- Connect your mark from Step 2 with the left endpoint. Make your line 2 in long, the length of the second side.
- Connect the two endpoints by drawing the third side.
Can you draw another triangle, with these measurements that looks different? The answer is NO. Only one triangle can be created from any given two lengths and the INCLUDED angle.
Side-Angle-Side (SAS) Triangle Congruence Postulate: If two sides and the included angle in one triangle are congruent to two sides and the included angle in another triangle, then the two triangles are congruent.
The markings in the picture are enough to say that .
Example A
What additional piece of information would you need to prove that these two triangles are congruent using the SAS Postulate?
a)
b)
c)
d)
For the SAS Postulate, you need two sides and the included angle in both triangles. So, you need the side on the other side of the angle. In , that is and in that is . The correct answer is c.
Example B
Write a two-column proof to show that the two triangles are congruent.
Given : is the midpoint of and
Prove :
Statement | Reason |
---|---|
1. is the midpoint of and | Given |
2. | Definition of a midpoint |
3. | Vertical Angles Postulate |
4. | SAS Postulate |
Example C
Is the pair of triangles congruent? If so, write the congruence statement and why.
While the triangles have two pairs of sides and one pair of angles that are congruent, the angle is not in the same place in both triangles. The first triangle fits with SAS, but the second triangle is SSA. There is not enough information for us to know whether or not these triangles are congruent.
Watch this video for help with the Examples above.
CK-12 Foundation: Chapter4SASTriangleCongruenceB
Vocabulary
Two figures are congruent if they have exactly the same size and shape. By definition, two triangles are congruent if the three corresponding angles and sides are congruent. The symbol means congruent. There are shortcuts for proving that triangles are congruent. The SAS Triangle Postulate states that if two sides and the included angle in one triangle are congruent to two sides and the included angle in another triangle, then the two triangles are congruent.
Guided Practice
1. Is the pair of triangles congruent? If so, write the congruence statement and why.
2. State the additional piece of information needed to show that each pair of triangles is congruent.
3. Fill in the blanks in the proof below.
Given :
Prove :
Statement | Reason |
---|---|
1. | 1. |
2. | 2. |
3. | 3. |
Answers:
1. The pair of triangles is congruent by the SAS postulate. .
2. We know that one pair of sides and one pair of angles are congruent from the diagram. In order to know that the triangles are congruent by SAS we need to know that the pair of sides on the other side of the angle are congruent. So, we need to know that .
3.
Statement | Reason |
---|---|
1. | 1. Given |
2. | 2. Vertical Angle Theorem |
3. | 3. SAS postulate |
Practice
Are the pairs of triangles congruent? If so, write the congruence statement and why.
State the additional piece of information needed to show that each pair of triangles is congruent.
- Use SAS
- Use SAS
- Use SAS
Complete the proofs below.
- Given : is a midpoint of Prove :
- Given : is an angle bisector of Prove :
- Given : is the midpoint of and is a right angle Prove :
- Given : is the angle bisector of Prove :
For each pair of triangles, write what needs to be congruent in order for the triangles to be congruent by SAS. Then, write the congruence statement for the triangles.