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Lesson Plan - Using Variables

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# Rectangle Perimeter 1

Alignments to Content Standards:
6.EE.A.2

## Task

To compute the perimeter of a rectangle you add the length, $l$ and width, $w$ and double this sum.

- Write an expression for the perimeter of a rectangle.
- Use the expression to ﬁnd the perimeter of a rectangle with length 30 and width 75.

## IM Commentary

This tasks gives a verbal description for computing the perimeter of a rectangle and asks the students to find an expression for this perimeter. They then have to use the expression to evaluate the perimeter for specific values of the two variables. In this problem, the variable names $l$ and $w$ convey the meaning of length and width of a rectangle. A follow up to this task is Rectangle Perimeter 2, where students are asked to determine if expressions for the perimeter of a rectangle are correct and equivalent.

## Attached Resources

## Solution

- The description for computing the perimeter of a rectangle first adds the length $l$ and the width $w$ of the rectangle $$l+w$$ Then it asks us to double this sum, that means we are taking the sum and multiplying it by $2$: $$\mbox{perimeter of a rectangle} = 2(l+w)$$
- Letting $l=30$ and $w=75$ we have $$\mbox{perimeter of this rectangle} = 2(30+75)=2(105) = 210.$$

## Rectangle Perimeter 1

To compute the perimeter of a rectangle you add the length, $l$ and width, $w$ and double this sum.

- Write an expression for the perimeter of a rectangle.
- Use the expression to ﬁnd the perimeter of a rectangle with length 30 and width 75.

## Comments

Log in to comment## phil daro says:

about 6 yearsA computes the perimeter of a rectangle by adding the length, l and width, w and double this sum. B computes the perimeter of a rectangle by doubling the length,l, doubling the width,w, and adding the doubled amounts. a. write an expression for A's way of calculating, and an expression for B's way. b. Use the expression to ﬁnd the perimeter of a rectangle with length 30 and width 75. c. show why A and B always get the same answer.