Perimeters and Areas of Rectangles

Lesson 9-3

Area

• Area is the amount of space inside a figure. We use square units to describe area.

• For rectangles, the area formula is A=lw (area equals length times width).

• For squares, the formula is still A=lw. However, since every side of a square is the same, then it can also be expressed as A=s2

where s equals the length of one side.

Example

12 m

8 m A=lwA=12(8)A=96 sq. m

Area is expressed in square units. It can be written (in this case) as 96 sq. m or 96 m2.

Area of Irregular ShapesCount how many squares are completely covered by the shape. Put partial squares together to make wholes.

•13 squares are full.

•2 squares are almost full, and they can be matched up with 2 squares that are almost empty.

•3 squares are about half full – when two are put together that makes a full square. The half square is left over.

•Altogether, there are 16 and a half squares full.

Perimeter

• Perimeter is the distance around the outside of a figure.

• The perimeter of a rectangle is derived by adding all four sides. Since the two lengths are equal, and the two widths are equal, then it can be written as P = 2l + 2w.

• For squares, the perimeter is found the same way (add all 4 sides). Since all sides are the same, however, you can use the formula P = 4s.

Example

18 cm

18 cm

5 cm 5 cm

P = 2w + 2lP = 2(5) + 2(18)P = 10 + 36P = 46 cm

Perimeter results in regular units, not square units.

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