Similar Figures

1
Similar Figures

• (Not exactly the same, but pretty close!)

2
Lets do a little review work before discussing
similar figures.
3
Congruent Figures

• In order to be congruent, two figures must be the
  same size and same shape.

4
Similar Figures

• Similar figures must be the same shape, but their
  sizes may be different.

5
Similar Figures

• This is the symbol that means similar.
• These figures are the same shape but different
  sizes.

6
SIZES

• Although the size of the two shapes can be
  different, the sizes of the two shapes must
  differ by a factor.

4
2
6
6
3
3
2
1
7
SIZES

• In this case, the factor is x 2.

4
2
6
6
3
3
2
1
8
SIZES

• Or you can think of the factor as 2.

4
2
6
6
3
3
2
1
9
Enlargements

• When you have a photograph enlarged, you make a
  similar photograph.

X 3
10
Reductions

• A photograph can also be shrunk to produce a
  slide.

4
11
Determine the length of the unknown side.
15
12
?
4
3
9
12
These triangles differ by a factor of 3.
15 3 5
15
12
?
4
3
9
13
Determine the length of the unknown side.
?
2
24
4
14
These dodecagons differ by a factor of 6.
?
2 x 6 12
2
24
4
15
Sometimes the factor between 2 figures is not
obvious and some calculations are necessary.
15
12
10
8
18
12
?
16
To find this missing factor, divide 18 by 12.
15
12
10
8
18
12
?
17
18 divided by 12 1.5
18
The value of the missing factor is 1.5.
15
12
10
8
18
12
1.5
19
When changing the size of a figure, will the
angles of the figure also change?
?
40
?
?
70
70
20
Nope! Remember, the sum of all 3 angles in a
triangle MUST add to 180 degrees.If the size of
theangles were increased,the sum would
exceed180degrees.
40
40
70
70
70
70
21
We can verify this fact by placing the smaller
triangle inside the larger triangle.
40
40
70
70
70
70
22
The 40 degree angles are congruent.
40
70
70
70
70
23
The 70 degree angles are congruent.
40
40
70
70
70
70
70
24
The other 70 degree angles are congruent.

4
40
70
70
70
70
70
25
Find the length of the missing side.
50
?
30
6
40
8
26
This looks messy. Lets translate the two
triangles.
50
?
30
6
40
8
27
Now things are easier to see.
50
30
?
6
40
8
28
The common factor between these triangles is 5.
50
30
?
6
40
8
29
So the length of the missing side is?
30
Thats right! Its ten!
50
30
10
6
40
8
31
Similarity is used to answer real life questions.

• Suppose that you wanted to find the height of
  this tree.

32
Unfortunately all that you have is a tape
measure, and you are too short to reach the top
of the tree.
33
You can measure the length of the trees shadow.
10 feet
34
Then, measure the length of your shadow.
10 feet
2 feet
35
If you know how tall you are, then you can
determine how tall the tree is.
6 ft
10 feet
2 feet
36
The tree must be 30 ft tall. Boy, thats a tall
tree!
6 ft
10 feet
2 feet
37
Similar figures work just like equivalent
fractions.
30
5
11
66
38
These numerators and denominators differ by a
factor of 6.
30
5
6
11
6
66
39
Two equivalent fractions are called a proportion.
30
5
11
66
40
Similar Figures

• So, similar figures are two figures that are the
  same shape and whose sides are proportional.

41
Practice Time!
42
1) Determine the missing side of the triangle.
?
9
5
?
3
4
12
43
1) Determine the missing side of the triangle.
15
9
5
?
3
4
12
44
2) Determine the missing side of the triangle.
36
36
6
6
?
4
?
45
2) Determine the missing side of the triangle.
36
36
6
6
?
4
24
46
3) Determine the missing sides of the triangle.
39
?
?
33
?
8
24
47
3) Determine the missing sides of the triangle.
39
13
?
33
11
8
24
48
4) Determine the height of the lighthouse.
?
?
8
2.5
10
49
4) Determine the height of the lighthouse.
?
32
8
2.5
10
50
5) Determine the height of the car.
?
?
3
5
12
51
5) Determine the height of the car.
7.2
?
3
5
12
52
THE END!