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Proportion and Similar
Triangles
Geometry
Lovemore

Proportion
• An equation stating that
two ratios are equal
– Example:
• Cross products: means
and extremes
– Example:
d
c
b
a

d
c
b
a
 a and d = extremes
b and c = means
ad = bc

Similar Polygons
• Similar polygons have:
• Congruent corresponding angles
• Proportional corresponding sides
• Scale factor: the ratio of corresponding sides
A
B
C D
E
L
M
N O
P
Polygon ABCDE ~ Polygon LMNOP
NO
CD
LM
AB

Ex:

Similar Triangles
• Similar triangles have
congruent
corresponding angles
and proportional
corresponding sides

A
B
C
Y
X
Z
ABC ~ XYZ
angle A angle X
angle B angle Y
angle C angle Z


YZ
BC
XZ
AC
XY
AB



Similar Triangles
• Triangles are similar if you show:
– Any 2 pairs of corresponding sides are
proportional and the included angles are
congruent (SAS Similarity)
A
B
C
R
S
T
18
12 6
4

Similar Triangles
– All 3 pairs of corresponding sides are
proportional (SSS Similarity)
A
B
C
R
S
T
10
14
6
7
5
3

Similar Triangles
– Any 2 pairs of corresponding angles are
congruent (AA Similarity)
A
B
C
R
S
T

Parts of Similar Triangles
• If two triangles are
similar, then the
perimeters are
proportional to the
measures of
corresponding sides
XZ
AC
YZ
BC
XY
AB
XYZ
perimeter
ABC
perimeter





A
B C
X
Y Z

• the measures of the
corresponding altitudes
are proportional to the
corresponding sides
• the measures of the
corresponding angle
bisectors are
proportional to the
corresponding sides
YZ
BC
YX
BA
XZ
AC
XW
AD



A
B C
X
Y Z
D
W
L
M
N
O
R
S
T
U
RT
LN
RS
LM
ST
MN
SU
MO



If two triangles are similar:

• If 2 triangles are similar,
then the measures of the
corresponding medians
are proportional to the
corresponding sides.
• An angle bisector in a triangle
cuts the opposite side into
segments that are proportional
to the other sides
G
H I
J
T
U V W
UW
HJ
TW
GJ
UT
GH
TV
GI



A
B
C
D
E
F
G
H
AD
AB
CD
BC

EH
EF
GH
FG


Theorem
Triangle Proportionality Theorem
If a line is parallel to one side of a
triangle and intersects the other two
sides, then it divides the two sides
proportionally.
EC
AE
DB
AD


Converse of the Triangle
Proportionally Theorem
If a line divides two sides of a triangle
proportionally, then the line is parallel
to the remaining side.
BC
DE
EC
AE
DB
AD
//



Parallel Lines and Proportional
Parts
• Triangle Midsegment
Theorem
– A midsegment of a
triangle is parallel to
one side of a triangle,
and its length is half of
the side that it is
parallel to
A
B
C
D
E
*If E and B are the midpoints
of AD and AC respectively,
then EB = DC
2
1

Theorem
If three parallel lines are intersected by
two lines, then the lines are divided
proportionally.

Parts
• If 3 or more lines are
parallel and intersect
two transversals, then
they cut the
transversals into
proportional parts
EF
DE
BC
AB

A
B
C
D
E
F
EF
BC
DF
AC

EF
DF
BC
AC


Parts
• If 3 or more parallel
lines cut off congruent
segments on one
transversal, then they
cut off congruent
segments on every
transversal
BC
AB 
A
B
C
D
E
F
EF
DE 
If , then

Theorem
If a ray bisects one angle of a triangle,
then it divides the sides proportional
with the sides they are touching.

Lets look at some worked examples

Solve for x
x
x
x




 3
6
)
9
(
2
9
6
2

Show that DE // BC
9
75
.
9
24
26

Show that DE // BC
9
75
.
9
24
26
BC
DE
EC
AE
DB
AD
//



Solve for x
9
6
8
x
x
x
x



3
1
5
9
48
8
9
6

Solve for x
20
15
17
x
 
7
2
7
255
35
15
255
20
17
15
20
20
17
15








x
x
x
x
x
x
x
x

A. Determine whether the triangles are similar. If so, write a
similarity statement. Explain your reasoning.

B. Determine whether the triangles are similar. If so, write a
similarity statement. Explain your reasoning.

A. Determine whether the triangles
are similar. If so, write a similarity
statement. Explain your reasoning.

B. Determine whether the triangles are
similar. If so, write a similarity
statement. Explain your reasoning.

A. Determine whether the triangles are
similar. If so, choose the correct similarity
statement to match the given data.

B. Determine whether the triangles are
similar. If so, choose the correct
similarity statement to match the given
data.

ALGEBRA Given , RS = 4, RQ = x + 3,
QT = 2x + 10, UT = 10, find RQ and QT.

SKYSCRAPERS Josh wanted to measure the height of the
Sears Tower in Chicago. He used a
12-foot light pole and measured its shadow at 1 p.m. The length
of the shadow was 2 feet. Then he measured the length of the
Sears Tower’s shadow and it
was 242 feet at the same time.
What is the height of the
Sears Tower?

In the figure, ΔABC ~ ΔFGH. Find the value of x.

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Similar Triangles PPT and examples.pptbbn

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