Math 7 (Period 4)

Writing & Solving Proportions 7.2

An equation that states tow ratios are equal is called a proportion.

The proportion  

 is read “ a is to b as c is to d.”

It has two cross
products ad and bc.

In words: In a proportion the cross products are equal

In Algebra: if 

 then ad = bc

With examples: because 2/5 = 4/10 you know that 2(10) = 4(5)

When you know three numbers in a proportion, you can find the missing value by using cross products

Use cross products property to solve the proportion  

3(15) = 5(m)
Now it is a one step equation
Divide both sides by 5

3(15)/5 = 5m/5
9 = m

Be sure to compare quantities in the same order in a proportion.

You are reading a novel—we talked about The Giver, one day you read 30 pages in 50 minutes. The next day you read 24  more pages in 40 minutes. Did you read at the same rate?

Setup a proportion

Does     

Notices on both the pages are in the numerator and the minutes are in the denominator.

Could we have written it  

 Yes.  Notices on both the pages are in the denominator now and the minutes are in the numerator.

We looked at the CORN BREAD RECIPE on Page 336

Amount	Ingredient	Directions
1  ¼ c	flour	Pre heat oven to 400 º
¾  c	Corn meal	Grease 8 by 8 inch pan
¼ c	sugar
2 t	Baking powder	Combine dry ingredients. Mix in milk,
1 c	Skim milk	oil and egg. Pour into pan. Bake 20
½ t	salt	to 25 minutes
¼ c	oil
1	Egg, beaten	9 servings

If you had 8 cups of corn meal, how many loaves of corn bread could you make?  Set up a proportion

  (3/4)x = 8  solve 

 x = 32/3 which is  10 2/3 so you could make 10 loaves

To make 10 loaves of corn bread, how much sugar would you need?

Again set up a proportion

  Solve 10(¼) = s   10/4 = s or 2½ cups of sugar