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.

A=lw A=12(8) A=96 sq. m Example 12 m 8 m Area is expressed in square units. It can be written (in this case) as 96 sq. m or 96 m2.

Area of Irregular Shapes Count 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.

P = 2w + 2l P = 2(5) + 2(18) P = 10 + 36 P = 46 cm Example 18 cm 5 cm Perimeter results in regular units, not square units. P = 10 + 36 P = 46 cm

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