geometry of triangles
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Chapter 9
Geometry
© 2008 Pearson Addison-Wesley.All rights reserved
© 2008 Pearson Addison-Wesley. All rights reserved9-4-2
Chapter 9: Geometry
9.1 Points, Lines, Planes, and Angles 9.2 Curves, Polygons, and Circles9.3 Perimeter, Area, and Circumference 9.4 The Geometry of Triangles: Congruence, Similarity,
and the Pythagorean Theorem9.5 Space Figures, Volume, and Surface Area9.6 Transformational Geometry9.7 Non-Euclidean Geometry, Topology, and Networks9.8 Chaos and Fractal Geometry
© 2008 Pearson Addison-Wesley. All rights reserved9-4-3
Chapter 1
Section 9-4The Geometry of Triangles:
Congruence, Similarity, and the Pythagorean Theorem
© 2008 Pearson Addison-Wesley. All rights reserved9-4-4
The Geometry of Triangles: Congruence, Similarity, and the Pythagorean Theorem
• Congruent Triangles• Similar Triangles • The Pythagorean Theorem
© 2008 Pearson Addison-Wesley. All rights reserved9-4-5
Congruent Triangles
A
B
C
Triangles that are both the same size and same shape are called congruent triangles.
D
E
FThe corresponding sides are congruent and corresponding angles have equal measures. Notation: .ABC DEF
© 2008 Pearson Addison-Wesley. All rights reserved9-4-6
Congruence Properties - SAS
Side-Angle-Side (SAS) If two sides and the included angle of one triangle are equal, respectively, to two sides and the included angle of a second triangle, then the triangles are congruent.
© 2008 Pearson Addison-Wesley. All rights reserved9-4-7
Congruence Properties - ASA
Angle-Side-Angle (ASA) If two angles and the included side of one triangle are equal, respectively, to two angles and the included side of a second triangle, then the triangles are congruent.
© 2008 Pearson Addison-Wesley. All rights reserved9-4-8
Congruence Properties - SSS
Side-Side-Side (SSS) If three sides of one triangle are equal, respectively, to three sides of a second triangle, then the triangles are congruent.
© 2008 Pearson Addison-Wesley. All rights reserved9-4-9
Example: Proving Congruence (SAS)
Given: CE = EDAE = EB
Prove: ACE BDE
STATEMENTS REASONS
1. CE = ED 1. Given
2. AE = EB 2. Given
3. 3. Vertical Angles are equal
4. 4. SAS property
A
B
DE
C
ACE BDE
Proof
CEA DEB
© 2008 Pearson Addison-Wesley. All rights reserved9-4-10
Example: Proving Congruence (ASA)
Given:
Prove: ADB CDB
STATEMENTS REASONS
1. 1. Given
2. 2. Given
3. DB = DB 3. Reflexive property
4. 4. ASA property
A
B
D
C
Proof
ADB CBD ABD CDB
ADB CDB
ADB CBD ABD CDB
© 2008 Pearson Addison-Wesley. All rights reserved9-4-11
Example: Proving Congruence (SSS)
Given: AD = CDAB = CB
Prove: ABD CDB
STATEMENTS REASONS
1. AD = CD 1. Given
2. AB = CB 2. Given
3. BD = BD 3. Reflexive property
4. 4. SSS property
A
B
D C
ABD CDB
Proof
© 2008 Pearson Addison-Wesley. All rights reserved9-4-12
Important Statements About Isosceles Triangles
If ∆ABC is an isosceles triangle with AB = CB, and if D is the midpoint of the base AC, then the following properties hold.
1. The base angles A and C are equal.2. Angles ABD and CBD are equal.3. Angles ADB and CDB are both right angles.
A C
B
D
© 2008 Pearson Addison-Wesley. All rights reserved9-4-13
Similar Triangles
Similar Triangles are pairs of triangles that are exactly the same shape, but not necessarily the same size. The following conditions must hold.
1. Corresponding angles must have the same measure.2. The ratios of the corresponding sides must be constant; that is, the corresponding sides are proportional.
© 2008 Pearson Addison-Wesley. All rights reserved9-4-14
Angle-Angle (AA) Similarity Property
If the measures of two angles of one triangle are equal to those of two corresponding angles of a second triangle, then the two triangles are similar.
© 2008 Pearson Addison-Wesley. All rights reserved9-4-15
Example: Finding Side Length in Similar Triangles
is similar to .ABC DEF Find the length of side DF.
A
B
CD
EF
Solution
1624
32
8
Set up a proportion with corresponding sides:
EF DFBC AC
816 32
DF Solving, we find that DF = 16.
© 2008 Pearson Addison-Wesley. All rights reserved9-4-16
Pythagorean Theorem
If the two legs of a right triangle have lengths a and b, and the hypotenuse has length c, then
That is, the sum of the squares of the lengths of the legs is equal to the square of the hypotenuse.
2 2 2.a b c
leg a
leg b
hypotenuse c
© 2008 Pearson Addison-Wesley. All rights reserved9-4-17
Example: Using the Pythagorean Theorem
Find the length a in the right triangle below.
Solution2 2 2a b c
39
36
a
2 2 236 39a 2 1296 1521a
2 225a 15a
© 2008 Pearson Addison-Wesley. All rights reserved9-4-18
Converse of the Pythagorean Theorem
If the sides of lengths a, b, and c, where c is the length of the longest side, and if
then the triangle is a right triangle.2 2 2 ,a b c
© 2008 Pearson Addison-Wesley. All rights reserved9-4-19
Example: Applying the Converse of the Pythagorean Theorem
Is a triangle with sides of length 4, 7, and 8, a right triangle?
Solution2 2 24 7 8
?
16 49 64 ?
65 64No, it is not a right triangle.
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