Transcript Similar

Similar Figures
Warm Up
Problem of the Day
Lesson Presentation
Lesson Quizzes
Similar Figures
Warm Up
Solve each proportion.
1. 3 = b
9
30
3. p = 4
9
12
b = 10
y = 56
2. 5
35
y=8
p=3
4. 28 = 56
m = 52
26
m
Similar Figures
Problem of the Day
A rectangle that is 10 in. wide and 8 in. long
is the same shape as one that is 8 in. wide
and x in. long. What is the length of the
smaller rectangle?
6.4 in.
Similar Figures
Learn to determine whether figures are
similar and to find missing dimensions in
similar figures.
Similar Figures
Vocabulary
similar
corresponding sides
corresponding angles
Similar Figures
Similar figures have the same shape, but not
necessarily the same size.
Corresponding sides of two figures are in the
same relative position, and corresponding angles
are in the same relative position. Two figures are
similar if the lengths of corresponding sides are
proportional and the corresponding angles have
equal measures.
Similar Figures
Similar Figures
Additional Example 1: Identifying Similar Figures
Which rectangles are similar?
Since the three figures are all rectangles, all the
angles are right angles. So the corresponding
angles are congruent.
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Additional Example 1 Continued
Compare the ratios of corresponding sides to see if they are equal.
length of rectangle J
length of rectangle K
10 ? 4
5 =2
20 = 20
width of rectangle J
width of rectangle K
The ratios are equal. Rectangle J is similar to
rectangle K. The notation J ~ K shows similarity.
length of rectangle J
length of rectangle L
10 ? 4
12 = 5
width of rectangle J
width of rectangle L
50  48
The ratios are not equal. Rectangle J is not similar to
rectangle L. Therefore, rectangle K is not similar to
rectangle L.
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Check It Out: Example 1
Which rectangles are similar?
8 ft
A
4 ft
6 ft
B
3 ft
5 ft
C
2 ft
Since the three figures are all rectangles, all the
angles are right angles. So the corresponding
angles are congruent.
Similar Figures
Check It Out: Example 1 Continued
Compare the ratios of corresponding sides to see if they are equal.
length of rectangle A
length of rectangle B
8 ? 4
6 =3
24 = 24
width of rectangle A
width of rectangle B
The ratios are equal. Rectangle A is similar to
rectangle B. The notation A ~ B shows similarity.
length of rectangle A
length of rectangle C
8 ? 4
5= 2
width of rectangle A
width of rectangle C
16  20
The ratios are not equal. Rectangle A is not similar
to rectangle C. Therefore, rectangle B is not similar
to rectangle C.
Similar Figures
Additional Example 2: Finding Missing Measures in
Similar Figures
A picture 10 in. tall and 14 in. wide is to be scaled to 1.5
in. tall to be displayed on a Web page. How wide should
the picture be on the Web page for the two pictures to be
similar?
Set up a proportion. Let w be the width of the picture on the
Web page.
width of a picture
width on Web page
14
10
=
w
1.5
height of picture
height on Web page
14 ∙ 1.5 = w ∙ 10
Find the cross products.
21 = 10w
21
10w
Divide both sides by 10.
=
10
10
2.1 = w
The picture on the Web page should be 2.1 in. wide.
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Check It Out: Example 2
A painting 40 in. long and 56 in. wide is to be scaled to
10 in. long to be displayed on a poster. How wide
should the painting be on the poster for the two
pictures to be similar?
Set up a proportion. Let w be the width of the painting on the
Poster.
56
40
width of a painting
length of painting
=
w
10
width of poster
length of poster
56 ∙ 10 = w ∙ 40
Find the cross products.
560 = 40w
560
40w
=
40
40
14 = w
Divide both sides by 40.
The painting displayed on the poster should be 14 in. long.
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Additional Example 3: Using Equivalent Ratios to
Find Missing Dimensions
A T-shirt design includes an isosceles triangle
with side lengths 4.5 in, 4.5 in., and 6 in. An
advertisement shows an enlarged version of the
triangle with two sides that are each 3 ft. long.
What is the length of the third side of the
triangle in the advertisement?
4.5 in. = 6 in.
3 ft
x ft
Set up a proportion.
4.5 • x = 3 • 6
Find the cross products.
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Additional Example 3 Continued
4.5x = 18
x = 18 = 4
4.5
Multiply.
Solve for x.
The third side of the triangle is 4 ft long.
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Check It Out: Example 3
A flag in the shape of an isosceles triangle with
side lengths 18 ft, 18 ft, and 24 ft is hanging on
a pole outside a campground. A camp t-shirt
shows a smaller version of the triangle with two
sides that are each 4 in. long. What is the length
of the third side of the triangle on the t-shirt?
18 ft = 24 ft
4 in.
x in.
Set up a proportion.
18 • x = 24 • 4
Find the cross products.
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Check It Out: Example 3 Continued
18x = 96
x = 96  5.3
18
Multiply.
Solve for x.
The third side of the triangle is about 5.3 in. long.
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Lesson Quizzes
Standard Lesson Quiz
Lesson Quiz for Student Response Systems
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Lesson Quiz
Use the properties of similar figures to answer
each question.
1. A rectangular house is 32 ft wide and 68 ft long.
On a blueprint, the width is 8 in. Find the length
on the blueprint. 17 in.
2. Karen enlarged a 3 in. wide by 5 in. tall photo into
a poster. If the poster is 2.25 ft wide, how tall is
it? 3.75 ft
3. Which rectangles are
similar?
A and B are similar.
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Lesson Quiz for Student Response Systems
1. A rectangular garden is 45 ft wide and 70 ft
long. On a blueprint, the width is 9 in. Identify the
length on the blueprint.
A. 7 in.
B. 9 in.
C. 12 in.
D. 14 in.
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Lesson Quiz for Student Response Systems
2. Rob enlarged a 4 in. wide by 7 in. tall picture
into a banner. If the banner is 3.5 ft wide, how tall
is it?
A. 6.05 ft
B. 6.125 ft
C. 7 ft
D. 7.125 ft
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Lesson Quiz for Student Response Systems
3. Which triangles are similar?
A. X and Y
Y
B. X and Z
C. Y and Z
D. none
X
Z