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**Mathematical Similarity**

Similar Figures & Scale Factor Similar Triangles

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Starter Questions Wednesday, 05 April 2017

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**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.

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**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.

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**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

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**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

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**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

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**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

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**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

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**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

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**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.

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Starter Questions Wednesday, 05 April 2017

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**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.

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**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?

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**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

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**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.

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**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.

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