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5.6 Inequalities in Two Triangles
Theorem 5.14: Hinge Theorem
_____WX ST
If two sides of one triangle are congruent to two sides of another triangle, and the included angle of the first is larger than the included angle of the second, then the third side of the first is _________ than the third side of the second.
longer
V
X
W
88R
S
T
35
5.6 Inequalities in Two Triangles
Theorem 5.15: Converse of the Hinge Theorem
_____Cm Fm
If two sides of one triangle are congruent to two sides of another triangle, and the third side of the first is longer than the third side of the second, then the included angle of the first is _________ than the included angle of the second.
larger
A
C B
12
D
EF
9
5.6 Inequalities in Two Triangles
Example 1 Use the Hinge Theorem and its converseUse the Hinge Theorem and its converse
Complete the statement with <, >, or =. Explain.
EF ____ AC a. CBD ____ ADB b. mmB
C
A61
E
D
F
62
A
D
B
C
34
33
a. You are given thatSolution
.____BC and ____AB Because 61o < ____, by the Hinge Theorem, AC < ____.
ED DFo62 EF
? ?
5.6 Inequalities in Two Triangles
Example 1 Use the Hinge Theorem and its converseUse the Hinge Theorem and its converse
Complete the statement with <, >, or =. Explain.
EF ____ AC a. CBD ____ ADB b. mmB
C
A61
E
D
F
62
A
D
B
C
34
33
b. You are given thatSolution
____AD by the Reflexive Property. Because 34 > 33, ____ > ____. So, by the Converse of the Hinge Theorem, ________ > ________.
BC BDAB CD
and you know that ____BD
ADBm CBDm
? ?
5.6 Inequalities in Two Triangles
Example 2 Solve a multi-step problemSolve a multi-step problem
TravelTravel Car A leaves a mall, heads due north for 5 mi and then turns due west for 3 mi. Car B leaves the same mall, heads due south for 5 mi and then turns 80o towards east for 3 mi. Which car is farther from the mall?Draw a diagram.
Mall
o80
o90
mi 5
mi 3
mi 5
mi 3
The distance driven and the distance back to the mall form two triangles, with _________ 5 mile sides and __________ 3 mile sides.
congruentcongruent
Add the third side to the triangle.Use linear pairs to find the included angles of _____ and _____.o90
o90
o100o100
Because 100o > 90o, Car ___ is farther from the mall than Car A by the _______________.
BHinge Theorem
B
A
5.6 Inequalities in Two Triangles
Example 3 Write an indirect proofWrite an indirect proof
Write an indirect proof to show that an odd number is not Write an indirect proof to show that an odd number is not divisible by 6divisible by 6.
Given x is an odd number.Prove x is not divisible by 6.
Assume temporarily that ______________________.
This means that ____ = n for some whole number n.
x is divisible by 6
6
xStep 1Step 1Solution
So, multiplying both sides by 6 gives ___ = ___. x 6n
5.6 Inequalities in Two Triangles
Example 3 Write an indirect proofWrite an indirect proof
Write an indirect proof to show that an odd number is not Write an indirect proof to show that an odd number is not divisible by 6divisible by 6.
Given x is an odd number.Prove x is not divisible by 6.
SolutionIf x is odd, then by definition, x cannot be divided evenly
by ___. However, __ = ___ so ___ = ____ = ___. 2 2
xStep 2Step 2
x 6n 2
6n3n
We know that __ is a whole number because n is a whole number, so x can be divided evenly by __.
3n2
This contradicts the given statement that _________. x is odd
Therefore, the assumption that x is divisible by 6 is _____,Step 3Step 3 false
which proves that _______________________.x is not divisible by 6
5.6 Inequalities in Two Triangles Checkpoint. Complete the following exercises. Checkpoint. Complete the following exercises.
1. CDB,ADB If mmwhich is longer, AB or CB?
A
B
DC91 89
longer is AB
5.6 Inequalities in Two Triangles Checkpoint. Complete the following exercises. Checkpoint. Complete the following exercises.
In example 2, car C leaves the mall, and goes 5 miles due west then turns 85o towards south for 3 mi. Write the cars in order from the car closest to the mall to the car farthest from the mall.
Mall
o80
o90
mi 5
mi 3
mi 5
mi 3
o90
o100 B
A
o85mi 3
mi 5o95
C
Car A is closest
Car C is next closest
Car B is farthest
5.6 Inequalities in Two Triangles Checkpoint. Complete the following exercises. Checkpoint. Complete the following exercises.
3. Suppose you want to prove the statement “If x + y = 5 and y = 2, then x = 3.” What temporary assumption could you make to prove the conclusion indirectly?
You can temporarily assume that
3x