1. In Exercises 1 and 2, quadrilateral WASH quadrilateral NOTE.
Congruent Figures
GEOMETRY LESSON 4-1
In Exercises 1 and 2, quadrilateral WASH quadrilateral NOTE.
1. List the congruent corresponding parts.
2. mO = mT = 90 and mH = 36. Find mN.
3. Write a statement of triangle congruence. 4. Write a statement of triangle congruence. 5. Explain your reasoning in Exercise 4 above.
WA NO, AS OT, SH TE, WH NE;
W N,
A O,
S T,
H E
144
Sample: DFH ZPR
Sample: ABD CDB
Sample: Two pairs of corresponding sides and two pairs of corresponding angles are given. C A because all right angles are congruent. BD BD by the Reflexive Property of ABD CDB by the definition of congruent triangles.
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2. Triangle Congruence by SSS and SAS
GEOMETRY LESSON 4-2
(For help, go to Lesson 2-5.)
What can you conclude from each diagram?
2. The two triangles share a side, so PR  PR. According to the tick marks on the angles, QPR  SRP and Q  S.
3. According to the tick marks on the sides, TO  NV. The tick marks on the angles show that M  S. Since MO || VS, by the Alternate Interior Angles Theorem MON  SVT. Since OV  OV by the Reflexive Property, you can use the Segment Addition Property to show TV  NO.
1. According to the tick marks on the sides, AB  DE. According to the tick marks on the angles, C  F.
Check Skills You’ll Need
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5. Triangle Congruence by SSS and SAS
GEOMETRY LESSON 4-2
The property of triangle rigidity gives you a shortcut for proving two triangles congruent. It states that if the side lengths of a triangle are given, the triangle can have only one shape.
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6. Triangle Congruence by SSS and SAS
GEOMETRY LESSON 4-2
For example, you only need to know that two triangles have three pairs of congruent corresponding sides. This can be expressed as the following postulate.
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7. Triangle Congruence by SSS and SAS
GEOMETRY LESSON 4-2
It can also be shown that only two pairs of congruent corresponding sides are needed to prove the congruence of two triangles if the included angles are also congruent.
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8. B is the included angle between sides AB and BC.
Triangle Congruence by SSS and SAS
GEOMETRY LESSON 4-2
An included angle is an angle formed by two adjacent sides of a polygon.
B is the included angle between sides AB and BC.
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9. Triangle Congruence by SSS and SAS
GEOMETRY LESSON 4-2
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10. Triangle Congruence by SSS and SAS
GEOMETRY LESSON 4-2
The letters SAS are written in that order because the congruent angles must be between pairs of congruent corresponding sides.
Caution
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11. NO! SSA S S A A S S SSA: Side-Side-Angle THIS DOES NOT WORK!
It is a very common trap and students fall for it easily.
Example: are these triangles congruent?
NO!
SSA S S A A S S

12. Why doesn’t it work?

13. Why doesn’t it work?
These triangles are not congruent even though two sides and an angle are congruent.

14. out of yourself. Don't use SSA!
Don't make an
out of yourself. Don't use SSA!

15. Decide if the following triangles are congruent using SSS or SAS.

16. Use SSS or SAS.
 by SSS X Y S S S S S Z W

17. Not  (uses SSA) E S S A S A M T A

18.  by SAS L S A S K J A S H

19.  by SAS A S S S A A K S S D T

20. Not  (uses SSA) E L S S A A S S O N V

21.  by SSS S S S S S

22. Summary
To prove two triangles are congruent use
SSS or
SAS

23. Triangle Congruence by SSS and SAS
GEOMETRY LESSON 4-2
Using SSS
Given: M is the midpoint of XY, AX AY
Prove: AMX AMY
Write a paragraph proof.
Copy the diagram. Mark the congruent sides.
You are given that M is the midpoint of XY, and AX AY. Midpoint M implies MX MY. AM AM by the Reflexive Property of Congruence, so AMX AMY by the SSS Postulate.
Quick Check
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24. Triangle Congruence by SSS and SAS
GEOMETRY LESSON 4-2
Using SAS
AD BC. What other information do you need to prove ADC BCD by SAS?
It is given that AD BC. Also, DC CD by the Reflexive Property of Congruence. You now have two pairs of corresponding congruent sides. Therefore if you know ADC BCD, you can prove ADC BCD by SAS.
Edit text. See page 62.
GEOM_3eTP04_58-74_MQ
Quick Check
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25. Triangle Congruence by SSS and SAS
GEOMETRY LESSON 4-2
Are the Triangles Congruent?
Quick Check
Given: RSG  RSH,
From the information given, can you prove RSG  RSH? Explain.
Copy the diagram. Mark what is given on the diagram.
It is given that RSG RSH and SG SH.
RS RS by the Reflexive Property of Congruence.
Two pairs of corresponding sides and their included angles are congruent, so RSG RSH by the SAS Postulate.
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26. Triangle Congruence by SSS and SAS
GEOMETRY LESSON 4-2
1. In VGB, which sides include B?
2. In STN, which angle is included between NS and TN?
3. Which triangles can you prove congruent?
Tell whether you would use the SSS or SAS Postulate.
4. What other information do you need to prove
DWO DWG?
5. Can you prove SED BUT from the information given? Explain.
BG and BV N
APB XPY; SAS
If you know DO DG, the triangles are by SSS; if you know DWO DWG, they are by SAS.
No; corresponding angles are not between corresponding sides.
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