Geometry Warm-Up 1/19/11
∆ABC ~ ∆EDF
E
1. Find the scale
factor.
B
8
105
A
D
15
45
10
C
F
2. Find the length
of DE.
3. Find the measure
of E.
Section 7.3 Similar Triangles
Geometry
January 19, 2011
Similar Polygons

Polygons are similar if:
1) The corresponding angles are congruent
2) The sides are proportional
Ex 1: In the diagram,
∆BAD ~ ∆EAC.
A

Write the statement of
proportionality.

Find the m∠AEC.

Find EA & BE.
34
20
B
E
C
3
79
D
12
Ex 2: In the diagram,
∆ LMN ~ ∆ PQN.

Write the statement
of proportionality.

Find the m∠ M &
m∠ P.

Find MN & QM.
33
M
L
106
N
20
Q
36
P
30
Angle-Angle Similarity Postulate

AA ~
– Two triangles are considered similar if 2
angles in one triangle are  to 2 angles in
another triangle.
M
C
B
L
A
K
Side-Side-Side Similarity
Postulate

Side-Side-Side
(SSS)- when the
corresponding sides
of two triangles are
proportional
P
A
Q
R
– AB = BC = CA
PQ QR RP
B
C
Side-Angle-Side Similarity
Postulate

Side-Angle-Side
(SAS)- when an angle
in one triangle is
congruent to an angle
in a second triangle
and the lengths of the
included sides are
proportional
– X=M and ZX = XY
PM MN
M
X
N
P
Z
Y
Which of the following triangles
are similar?
12
G
H
6
9
14
A
C
I
6
10
J
4
6
L
B
K
8
Ex 3: Low-level aerial photos can be
taken using a remote-controlled camera
suspended from a blimp. You want to
take an aerial photo that covers a ground
distance g of 50 meters. Use the
proportion f/h = n/g to estimate the
altitude h that the blimp should fly at to
take the photo. In the proportion use
f=8cm and n=3cm. These two variables
are determined by the type of camera
used.
Ex 4: Find the length of the
altitude QS.
N
12
12
M
P
6
Q
R
8
S
T
8
HOMEWORK:
 P.
401 (7-15)
Geometry

1/20/11
Choose one problem from the
homework for your group to present.
Once you have selected the problem,
write the number on the board with the
name of someone from your group!