Ratios and proportions can be used to compare quantities. A proportion exists when two ratios are equivalent, meaning the quantities have an equal relationship. To check if two ratios form a proportion, you can use cross multiplication or the scale factor method. Cross multiplication checks if the products of the means and extremes are equal. The scale factor method identifies a common factor that makes the ratios equal when applied. Homework involves copying proportions and showing work checking for equivalency using these methods.

1. Ratios and Proportions

2. Definitions
• A proportion is two ratios that are
equivalent
• A ratio is a comparison of two items
• Equivalent means equal

3. How do you write equivalent ratios?
102
1
=
48
9
24
=

4. To check if 2 ratios are
proportional…you can use cross
products or cross multiplying
5
2
10
4
=
Is 2 x 10 equal to 4 x 5?
If yes, you have a proportion!

5. You try it! Is this a proportion?
8
6
12
9
=

6. You can also use “scale factor” to
see if two ratios form a proportion
8
3
24
9
=
x 3 =
x 3 =
Since 3 x 3 = 9 and 8 x 3 = 24, these
ratios do form a proportion! 3 is called
the “scale factor”.

7. You Try it! Is this a proportion?
10
7
15
14
=

8. You Try it! Is this a proportion?
5
12
15
36
=

9. Homework:
P. 240 #1-9 all; 10-28 evens
Copy the proportions and show your work.