6.2.1 Use Proportions to solve Geometry Problems

For any proportion, the reciprocal is also true D

C

B

A C

D

A

B

• Write and equivalent proportion:

x

x

5

1

10

2

5

1

10

2

1

5

2

10

1

5

2

10 x

x

• Cross multiply to check your work:

1*105*2

5*21*10

10*1)5(2 xx

)5(21*10 xx

For any proportion the means can be interchanged to make an equivalent proportion

D

C

B

A

D

B

C

A

• Write an equivalent proportion using the mean interchange property

5

1

10

2 x

x

5

1

10

2

• Cross multiply to check your work:

For any proportion the denominators can be added to the numerators to make an equivalent proportion

D

C

B

A

D

DC

B

BA

• Write an equivalent proportion using the Proportional Addition Property

5

1

10

2

x

x

5

1

10

2

• Cross multiply to check your work:

A scale is when something is drawn based on a ratio

Example: in real life my car is 17 ½ feet long, what is the scale based on the model? (show model car to class)

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