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10.1: Area and Perimeter of Rectangles

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What if Ed’s parents were getting him a new bed, and he had to decide what size bed is best for him? Initially he decided that he would like a king bed. Upon further research, Ed discovered there are two types of king beds, an Eastern (or standard) King and a California King. The Eastern King has 76'' \times 80'' dimensions, while the California King is 72'' \times 84'' (both dimensions are width \times length ). Which bed has a larger area to lie on? Which one has a larger perimeter? If Ed is 6’4”, which bed makes more sense for him to buy? After completing this Concept, you'll be able to use your knowledge of rectangles to answer these questions.

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CK-12 Foundation: Chapter10AreaandPerimeterofRectanglesA

Math Made Easy: Finding the Area of Rectangles

Guidance

To find the area of a rectangle calculate the product of its base (width) and height (length) A=bh . The perimeter of a rectangle is P=2b+2h , where b is the base (or width) and h is the height (or length). If a rectangle is a square , with sides of length s , the formula for perimeter is P_{square}=2s+2s=4s and the formula for area is A_{square}=s \cdot s=s^2 .

Example A

Find the area and perimeter of a rectangle with sides 4 cm by 9 cm.

The perimeter is 4 + 9 + 4 + 9 = 26 \ cm . The area is A=9 \cdot 4=36 \ cm^2 .

Example B

The area of a square is 75 \ in^2 . Find the perimeter.

To find the perimeter, we need to find the length of the sides.

A = s^2 & =75 \ in^2\\s &= \sqrt{75}=5 \sqrt{3} \ in\\\text{From this}, \ P& =4 \left( 5 \sqrt{3} \right)=20 \sqrt{3} \ in.

Example C

Find the area and perimeter of a rectangle with sides 13 m and 12 m.

The perimeter is 2(13)+2(12)=50 \ m . The area is 13(12)=156 \ m^2 .

Watch this video for help with the Examples above.

CK-12 Foundation: Chapter10AreaandPerimeterofRectanglesB

Concept Problem Revisited

The area of an Eastern King is 6080 \ in^2 and the California King is 6048 \ in^2 . The perimeter of both beds is 312 in. Because Ed is 6’4”, he should probably get the California King because it is 4 inches longer.

Vocabulary

Perimeter is the distance around a shape. The perimeter of any figure must have a unit of measurement attached to it. If no specific units are given (feet, inches, centimeters, etc), write “units.” Area is the amount of space inside a figure. Area is measured in square units.

Guided Practice

1. Find the area and perimeter of a square with side 5 in.

2. Draw two different rectangles with an area of 36 \ cm^2 .

3. Find the area and perimeter of a rectangle with sides 7 in and 10 in.

Answers:

1. The perimeter is 4(5)=20in and the area is 5^2=25 \ in^2 .

2. Think of all the different factors of 36. These can all be dimensions of the different rectangles.

Other possibilities could be 6 \times 6, 2 \times 18 , and 1 \times 36 .

3. Area is 7(10)=70 \ in^2 . Perimeter is 2(7)+2(10)=34 \ in .

Interactive Practice

Practice

  1. Find the area and perimeter of a square with sides of length 12 in.
  2. Find the area and perimeter of a rectangle with height of 9 cm and base of 16 cm.
  3. Find the area and perimeter of a rectangle if the height is 8 and the base is 14.
  4. Find the area and perimeter of a square if the sides are 18 ft.
  5. If the area of a square is 81 \ ft^2 , find the perimeter.
  6. If the perimeter of a square is 24 in, find the area.
  7. The perimeter of a rectangle is 32. Find two different dimensions that the rectangle could be.
  8. Draw two different rectangles that haven an area of 90 \ mm^2 .
  9. True or false: For a rectangle, the bigger the perimeter, the bigger the area.
  10. Find the perimeter and area of a rectangle with sides 17 in and 21 in.

For problems 11 and 12 find the dimensions of the rectangles with the given information.

  1. A rectangle with a perimeter of 20 units and an area of 24 \ units^2 .
  2. A rectangle with a perimeter of 72 units and an area of 288 \ units^2 .
  3. A rectangle with perimeter 138 units is divided into 8 congruent rectangles as shown in the diagram below. Find the perimeter and area of one of the 8 congruent rectangles.
  4. The length of a rectangle is 2 more than 3 times the width. The perimeter of the rectangle is 44 units. What is the area of the rectangle?
  5. The length of a rectangle is 2 less than 2 times the width. The area of the rectangle is 84 \ units^2 . What is the perimeter of the rectangle?

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Difficulty Level:

At Grade

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Date Created:

Jul 17, 2012

Last Modified:

Aug 21, 2014
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