Lesson: What is a proportion?

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Lesson Objective

We will be able to complete proportions using cross multiplication.

Lesson Plan

Do Now: (5 minutes)

1) Every 4th student in the cafeteria will receive an ice cream sandwich and every 6th student will receive a cookie. Which student will be the first to receive both an ice cream sandwich and a cookie?
2) 35 ÷ 0.7
3) Evaluate 3ab - 2c if a = 4, b = 3 and c = 5

Direct Instruction:(20 minutes)

This summer I went to Scotland and got to see some Highland cattle. Highland cattle are cows that live in the mountains in Scotland. Because it can get so cold and snowy there, they have more fur than the cattle that we are used to seeing. I want to show you some pictures of Hamish, a hairy cow that I got to meet. What are some of the things that we notice about these pictures?
 Show the four pictures of Hamish.  One is the original, one is only enlarged vertically, one enlarged only horizontally and one is enlarged proportionally.
Discuss why the pictures may or may not look right. Discuss what makes something proportional.
Here is an example of a proportion. How do you think we can check to make sure that it is a proportion?
Ex. 300/1200 = ¼
Proportions have equal cross-products.
Ex. A carousel makes 4 complete turns after 64 seconds and 5 complete turns after 76 seconds. Is the number of turns proportional to the time in seconds?
To complete a proportion:
1) Find the cross products
2) Solve for the variable

Guided Practice: (10 minutes)
Ex. 21/5 = x /7
Ex. For every 18 people who have a sore throat, there are two people that actually have strep throat. If 72 patients have sore throats, how many of these would you expect to have strep throat? Justify your answer.

Assessment: (10 minutes)
Proportions work-out

Lesson Resources

proportions work out   Assessment


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