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Area and Perimeter of Rectangles

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What if you were given a rectangle and the size of its base and height? How could you find the total distance around the rectangle and the amount of space it takes up? After completing this Concept, you'll be able to use the formulas for the perimeter and area of a rectangle to solve problems like this.

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Area and Perimeter of Rectangles CK-12

Guidance

To find the area of a rectangle, calculate A=bh , where b is the base (width) and h is the height (length). The perimeter of a rectangle will always be P=2b+2h .

If a rectangle is a square, with sides of length s , then perimeter is P_{square}=2s+2s=4s and area is A_{sqaure}=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

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

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

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 .

Area and Perimeter of Rectangles CK-12

Guided Practice

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

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. 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

From this, P=4 \left (5\sqrt{3} \right )=20\sqrt{3} \ in .

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 .

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.

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