4-3
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Transcript of 4-3
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4-3 Triangle Congruence: SSS and SAS
Holt Geometry
Warm UpWarm Up
Lesson PresentationLesson Presentation
Lesson QuizLesson Quiz
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Warm Up
1. Name the angle formed by AB and AC.
2. Name the three sides of ABC.
3. ∆QRS ∆LMN. Name all pairs of congruent corresponding parts.
Possible answer: A
QR LM, RS MN, QS LN, Q L, R M, S N
AB, AC, BC
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Apply SSS and SAS to construct triangles and solve problems.
Prove triangles congruent by using SSS and SAS.
Objectives
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triangle rigidityincluded angle
Vocabulary
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In First part of Lesson 4-3, you proved triangles congruent by showing that all six pairs of corresponding parts were congruent.
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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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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Adjacent triangles share a side, so you can apply the Reflexive Property to get a pair of congruent parts.
Remember!
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Example 1: Using SSS to Prove Triangle Congruence
Use SSS to explain why ∆ABC ∆DBC.
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Statement Reason
Given (Side)
Given (Side)
Reflexive Prop. (Side)
By SSS
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Check It Out! Example 1
Use SSS to explain why ∆ABC ∆CDA.
4.3 Congruent Triangles by SSS - SAS
Statement Reason
Given (Side)
Given (Side)
Reflexive Prop. (Side)
By SSS
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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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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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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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Example 2: Engineering Application
The diagram shows part of the support structure for a tower. Use SAS to explain why ∆XYZ ∆VWZ.
4.3 Congruent Triangles by SSS - SAS
Statement Reason
Given (Side)
Vertical <s (Angle)
Given (Side)
By SAS
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Example 2: Engineering Application
You can also write a paragraph proving that the two triangle are congruent by a side, angle, side. (SAS) (Notice the angle is in between the two sides).
It is given that XZ VZ and that YZ WZ. By the Vertical s Theorem. XZY VZW. Therefore ∆XYZ ∆VWZ by SAS.
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Check It Out! Example 2
Use SAS to explain why ∆ABC ∆DBC.
4.3 Congruent Triangles by SSS - SAS
Statement Reason
Given (Side)
Given (Angle)
Reflexive Prop. (Side)
By SAS
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Check It Out! Example 2
Using a paragraph.
It is given that BA BD and ABC DBC. By the Reflexive Property of , BC BC. So ∆ABC ∆DBC by SAS.
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The SAS Postulate guarantees that if you are given the lengths of two sides and the measure of the included angles, you can construct one and only one triangle.
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Example 3A: Verifying Triangle Congruence
Show that the triangles are congruent for the given value of the variable.
∆MNO ∆PQR, when x = 5.
∆MNO ∆PQR by SSS.
PQ = x + 2
= 5 + 2 = 7
PQ MN, QR NO, PR MO
QR = x = 5
PR = 3x – 9
= 3(5) – 9 = 6
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Example 3B: Verifying Triangle Congruence
∆STU ∆VWX, when y = 4.
∆STU ∆VWX by SAS.
ST = 2y + 3
= 2(4) + 3 = 11
TU = y + 3
= 4 + 3 = 7
mT = 20y + 12
= 20(4)+12 = 92°ST VW, TU WX, and T W.
Show that the triangles are congruent for the given value of the variable.
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Check It Out! Example 3
Show that ∆ADB ∆CDB, t = 4.
DA = 3t + 1
= 3(4) + 1 = 13
DC = 4t – 3
= 4(4) – 3 = 13
mD = 2t2
= 2(16)= 32°
∆ADB ∆CDB by SAS.
DB DB Reflexive Prop. of .
ADB CDB Def. of .
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Example 4: Proving Triangles Congruent
Given: BC ║ AD, BC ADProve: ∆ABD ∆CDB
ReasonsStatements
5. SAS Steps 3, 2, 45. ∆ABD ∆ CDB
4. Reflex. Prop. of
3. Given
2. Alt. Int. s Thm.2. CBD ABD
1. Given1. BC || AD
3. BC AD
4. BD BD
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Check It Out! Example 4
Given: QP bisects RQS. QR QS
Prove: ∆RQP ∆SQP
ReasonsStatements
5. SAS Steps 1, 3, 45. ∆RQP ∆SQP
4. Reflex. Prop. of
1. Given
3. Def. of bisector3. RQP SQP
2. Given2. QP bisects RQS
1. QR QS
4. QP QP
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Lesson Quiz: Part I
1. Show that ∆ABC ∆DBC, when x = 6.
ABC DBCBC BCAB DB
So ∆ABC ∆DBC by SAS
Which postulate, if any, can be used to prove the triangles congruent?
2. 3.none SSS
26°
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Lesson Quiz: Part II
4. Given: PN bisects MO, PN MO
Prove: ∆MNP ∆ONP
1. Given2. Def. of bisect3. Reflex. Prop. of 4. Given5. Def. of 6. Rt. Thm.7. SAS Steps 2, 6, 3
1. PN bisects MO2. MN ON3. PN PN4. PN MO 5. PNM and PNO are rt. s6. PNM PNO
7. ∆MNP ∆ONP
Reasons Statements
4.3 Congruent Triangles by SSS - SAS