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Comparing Measures of a Triangle
There is a relationship between the positions of the longest and shortest
sides of a triangle and the positions of its angles.
largest angle
shortest side
longest side
smallest angle
Comparing Measures of a Triangle
There is a relationship between the positions of the longest and shortest
sides of a triangle and the positions of its angles.
SIDE / ANGLE COMPARISON THEOREMS
Theorem 5.10
If one side of a triangle is longer than
another side, then the angle opposite the
longer side is larger than the angle
opposite the shorter side.
Theorem 5.11
If one angle of a triangle is larger than
another angle, then the side opposite the
larger angle is longer than the side
opposite the smaller angle.
B
5
3
A
C
m A > m C
D
60˚
F
40˚
E
EF > DF
Writing Measurements in Order from Least to Greatest
Write the measurements of the triangles in order from least to greatest.
J
100˚
H
45˚
35˚
G
SOLUTION
mG < m H < mJ
JH < JG < GH
Writing Measurements in Order from Least to Greatest
Write the measurements of the triangles in order from least to greatest.
J
8
100˚
H
45˚
5
35˚
G
SOLUTION
mG < m H < mJ
JH < JG < GH
R
Q
7
P
SOLUTION
QP < PR < QR
m R < mQ < m P
Comparing Measurements of a Triangle
Theorem 5.10 can be proved using the diagram shown below.
Given
AC > AB
Prove
mABC > mC
A
2
Paragraph Proof
B
D
1
3
Use the Ruler Postulate to locate a point D on AC such that
DA = BA. Then draw the segment BD.
In the isosceles triangle ABD, 1 2. Because mABC = m1 + m3, it
follows that mABC > m1.
Substituting m2 for m1 produces mABC > m 2.
Because m2 = m3 + mC, m2 > mC.
Finally, because mABC > m2 and m2 > mC, you can conclude that
mABC > mC.
C
Comparing Measurements of a Triangle
The proof of Theorem 5.10 used an exterior angle and showed that its
measure is the sum of the measures of two nonadjacent interior angles.
Then the measure of the exterior angle must be greater than the measure of
either nonadjacent interior angle.
THEOREM
Theorem 5.12 Exterior Angle Inequality
A
The measure of an exterior angle of a triangle is
greater than the measure of either of the two
nonadjacent interior angles.
m1 > mA and m1 > mB
1
C
B
Using the Triangle Inequality
Not every group of three segments can be used to form a triangle. The
lengths of the segments must fit a certain relationship.
Construct a triangle with the given group of side lengths, if possible.
2 cm, 2 cm, 5 cm
SOLUTION
Try drawing triangles with the given side lengths.
Impossible
Using the Triangle Inequality
Not every group of three segments can be used to form a triangle. The
lengths of the segments must fit a certain relationship.
Construct a triangle with the given group of side lengths, if possible.
2 cm, 2 cm, 5 cm
SOLUTION
3 cm, 2 cm, 5 cm
Try drawing triangles with the given side lengths.
Impossible
Impossible
Using the Triangle Inequality
Not every group of three segments can be used to form a triangle. The
lengths of the segments must fit a certain relationship.
Construct a triangle with the given group of side lengths, if possible.
2 cm, 2 cm, 5 cm
SOLUTION
3 cm, 2 cm, 5 cm
4 cm, 2 cm, 5 cm
Try drawing triangles with the given side lengths.
Impossible
Impossible
Only the last group is possible.
The sum of the first and second lengths must be greater than the third length.
Using the Triangle Inequality
The result of the previous example is summarized as Theorem 5.13.
THEOREM
Theorem 5.13 Triangle Inequality
A
The sum of the lengths of any two sides of a triangle is
greater than the length of the third side.
AB + BC > AC
AC + BC > AB
AB + BC > BC
C
B
Finding Possible Side Lengths
A triangle has one side of 10 centimeters and another of 14 centimeters.
Describe the possible lengths of the third side.
SOLUTION
Let x represent the length of the third side.
Using the Triangle Inequality, you can write and solve inequalities.
x + 10 > 14
x > 4
10 + 14 > x
24 > x
So, the length of the third side must be greater than 4 centimeters and less
than 24 centimeters.
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