Module 9 Lesson 3 Perimeter of a Rectangle

Module 9: Apply Multiplication To Area

Lesson 3: Find Unknown Measures pages 227-229

We Can..................

• find the unknown measure of a rectangle given the length of one side and the area or perimeter.

• Solve problems involving unknown measures in rectangles.

• Explain how to use the area formula to find the unknown measure of a rectangle given the length of one side and the area.

• Explain how to use the perimeter formula to find the unknown measure of a rectangle given the length of one side and the perimeter.

• Solve problems about the area and perimeter of rectangles and explain reasoning.

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Perimeter

Perimeter is the distance around a two-dimensional shape.

Example: the perimeter of this rectangle is 7+3+7+3 = 20

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Multiple Choice

Problem of the Day

The perimeter of a rectangle is 20 inches. What are possible measurements for the length and width? Justify your answer.

• 2 inches and 8 inches;

  4x3+6x6=6+12=22

• 3 inches and 7 inches;

  2x3+2x7=6+14=20

• 4 inches and 6 inches;

  6x3+2x7=6+13=21

• 5 inches and 4 inches;

  5x3+2x7=6+11=23

Multiple Choice

I can find the unknown measure of a rectangle given the length of one side and the area or perimeter.

Leisl is decorating the front window of a shop with a string of lights. She uses 50 feet of lights around the window. The window is 10 feet wide. What is the length of the window?

• The window is 11 feet wide. The perimeter, or distance around the window is 40 feet.

• The window is 12 feet wide. The perimeter, or distance around the window is 30 feet.

• The window is 10 feet wide. The perimeter, or distance around the window is 50 feet.

• The window is 13 feet wide. The perimeter, or distance around the window is 60 feet.

Fill in the Blanks

Use the perimeter formula to find the unknown. Write l for the unknown.

P = 2 × l + 2 × w

Fill in the Blanks

2 × l is an unknown addend. How can you use the equation to find 2 × l?

2xl=?

2x?=30 (what number multiplied by two is thirty?)

l=?

​The length of the window is 15 feet.

Multiple Choice

Step it Out page 228

Samuel is decorating a different window in the shop. The window is a rectangle and has an area of 72 square feet. The length of the window is 9 feet. What is the width? What do you want to find out?

• the length of the window.

• the height of the window.

• the interior of the window.

• the width of the window.

​What do you know? The window is nine feet long. The window has an area of 72 square feet.

Fill in the Blanks

Use the area formula to find the unknown. Write w for the unknown measure.

A = l × w

= _× w

What number multiplied by 9 is 72?

w = _

​The width of the window is eight feet.

Multiple Choice

Check Understanding

Use the area or perimeter formula to find the unknown measure.

1Elise is painting a stage set that has an area of 48 square feet. The width of the set is 6 feet. What is the length?

• 6 feet

• 7 feet

• 8 feet

• 9 feet

Multiple Choice

Check Understanding

Use the area or perimeter formula to find the unknown measure.

Jada has 38 yards of fencing to put around a paddock for some goats. She has the length set as 10 yards. What is the width of the paddock?

• 6 yards

• 9 yards

• 7 yards

• 8 yards

On Your Own​

Complete page 229 in your math workbook on your own.