1. Mathematical Similarity
Similar Figures & Scale Factor
Similar Triangles

2. Starter Questions
Wednesday, 05 April 2017

3. Created by Mr. Lafferty Maths Dept.
Similar Figures
Learning Intention
Success Criteria
To explain how the scale factor applies to similar figures.
1. Understand how the scale factor applies to similar figures.
2. Solve problems using scale factor.
5-Apr-17
Created by Mr. Lafferty Maths Dept.

4. Conditions for similarity
Two shapes are similar only when:
Corresponding sides are in proportion and
Corresponding angles are equal
All rectangles are not similar to one another since only condition 2 is true.

5. If two objects are similar then one is an enlargement of the other
The rectangles below are similar:
Find the scale factor of enlargement that maps A to B A B
8 cm
16 cm
5 cm
10 cm
Not to scale!
Scale factor = x2
Note that B to A
would be x ½

6. If two objects are similar then one is an enlargement of the other
The rectangles below are similar:
Find the scale factor of enlargement that maps A to B A B
8 cm
12 cm
5 cm
7½ cm
Not to scale!
Scale factor = x1½
Note that B to A
would be x 2/3

7. The 3 rectangles are similar. Find the unknown sides
8 cm
If we are told that two objects are similar and we can find the scale factor of enlargement then we can calculate the value of an unknown side. A B C
3 cm
Not to scale!
24 cm
x cm
y cm
13½ cm
The 3 rectangles are similar. Find the unknown sides
Comparing corresponding sides in A and B: 24/8 = 3 so x = 3 x 3 = 9 cm
Comparing corresponding sides in A and C: 13½ /3 = 4½ so y = 4 ½ x 8 = 36 cm

8. The 3 rectangles are similar. Find the unknown sides
5.6 cm
If we are told that two objects are similar and we can find the scale factor of enlargement then we can calculate the value of an unknown side. A B C
2.1 cm
Not to scale!
x cm
7.14 cm
26.88 cm
y cm
The 3 rectangles are similar. Find the unknown sides
Comparing corresponding sides in A and B: 7.14/2.1 = 3.4 so x = 3.4 x 5.6 = cm
Comparing corresponding sides in A and C: 26.88/5.6 = 4.8 so y = 4.8 x 2.1 = cm

9. Scale Factor applies to ANY SHAPES that are mathematically similar.
1.4 cm
7 cm
2cm y z
3 cm
15 cm
6 cm
Given the shapes are similar, find the values y and z ? 3 15 5 1
Scale factor =
ESF =
= 5
Scale factor =
RSF =
= 0.2 y
is 2 x 5 = 10 z
is 6 x 0.2 = 1.2

10. Scale Factor applies to ANY SHAPES that are mathematically similar.
8 cm
7.5 cm b
10 cm
4 cm
2cm a
3 cm
Given the shapes are similar, find the values a and b ? 10 4 4 10
Scale factor =
RSF =
= 0.4
Scale factor =
ESF =
= 2.5 a
is 8 x 0.4 = 3.2 b
is 2 x 2.5 = 5

11. Created by Mr. Lafferty Maths Dept.
Similar Figures
Now try Ex 1
Ch 9 (page 103)
5-Apr-17
Created by Mr. Lafferty Maths Dept.

12. Starter Questions
Wednesday, 05 April 2017

13. Created by Mr. Lafferty Maths Dept.
Similar Triangles
Learning Intention
Success Criteria
To explain how the scale factor applies to similar triangles.
1. Understand how the scale factor applies to similar triangles.
2. Solve problems using scale factor.
5-Apr-17
Created by Mr. Lafferty Maths Dept.

14. Scale factors www.mathsrevision.com Enlargement Scale factor? 3 2
ESF = = 8 8
8cm 12
12cm 12
Reduction Scale factor? 2 3
5cm
RSF = =
7.5cm
Can you see the relationship
between the two scale factors?

15. Scale factors a 9cm 27cm www.mathsrevision.com 5cm b 15cm
Find a given ESF = 3 a 9
ESF = 3 =
9cm
27cm a
By finding the RSF
Find the value of b.
5cm b 1 3 b 15 5 15
RSF = = =
15cm

16. Finding Unknown sides (1)
20 cm
18 cm
12 cm
6 cm b c
Since the triangles are equiangular they are similar.
So comparing corresponding sides.

17. Created by Mr. Lafferty Maths Dept.
Similar Triangles
Now try Ex 2
Ch 9 (page 107)
5-Apr-17
Created by Mr. Lafferty Maths Dept.