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SAS Triangle Congruence

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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 \overline{AB} and \overline{BC} are the given sides, so \angle B 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 45^\circ , can I construct only one triangle?

Investigation: Constructing a Triangle Given Two Sides and Included Angle

Tools Needed: protractor, pencil, ruler, and paper

  1. Draw the longest side (5 in) horizontally, halfway down the page. The drawings in this investigation are to scale.
  2. At the left endpoint of your line segment, use the protractor to measure a 45^\circ angle. Mark this measurement.
  3. Connect your mark from Step 2 with the left endpoint. Make your line 2 in long, the length of the second side.
  4. 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 \triangle ABC \cong \triangle XYZ .

Example A

What additional piece of information would you need to prove that these two triangles are congruent using the SAS Postulate?

a) \angle ABC \cong \angle LKM

b) \overline{AB} \cong \overline{LK}

c) \overline{BC} \cong \overline{KM}

d) \angle BAC \cong \angle KLM

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 \triangle ABC , that is \overline{BC} and in \triangle LKM that is \overline{KM} . The correct answer is c.

Example B

Write a two-column proof to show that the two triangles are congruent.

Given : C is the midpoint of \overline{AE} and \overline{DB}

Prove : \triangle ACB \cong \triangle ECD

Statement Reason
1. C is the midpoint of \overline{AE} and \overline{DB} Given
2. \overline{AC} \cong \overline{CE}, \overline{BC} \cong \overline{CD} Definition of a midpoint
3. \angle ACB \cong \angle DCE Vertical Angles Postulate
4. \triangle ACB \cong \triangle ECD 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 \cong 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 :

\overline{AB} \cong \overline{DC}, \ \overline{BE} \cong \overline{CE}

Prove : \triangle ABE \cong \triangle ACE

Statement Reason
1. 1.
2. \angle{AEB} \cong \angle{DEC} 2.
3. \triangle ABE \cong \triangle ACE 3.

Answers:

1. The pair of triangles is congruent by the SAS postulate.  \triangle CAB \cong \triangle QRS .

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 \overline{EF} \cong \overline{BA} .

3.

Statement Reason
1. \overline{AB} \cong \overline{DC}, \ \overline{BE} \cong \overline{CE} 1. Given
2. \angle{AEB} \cong \angle{DEC} 2. Vertical Angle Theorem
3. \triangle ABE \cong \triangle ACE 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.

  1. Use SAS
  2. Use SAS
  3. Use SAS

Complete the proofs below.

  1. Given : B is a midpoint of \overline{DC} \overline{AB} \perp \overline{DC} Prove : \triangle ABD \cong \triangle ABC
  2. Given : \overline{AB} is an angle bisector of \angle{DAC} \overline{AD} \cong \overline{AC} Prove : \triangle ABD \cong \triangle ABC
  3. Given : B is the midpoint of \overline{DE} and \overline{AC} \angle{ABE} is a right angle Prove : \triangle ABE \cong \triangle CBD
  4. Given : \overline{DB} is the angle bisector of \angle{ADC} \overline{AD} \cong \overline{DC} Prove : \triangle ABD \cong \triangle CBD

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.

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