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Perimeter and Area

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Perimeter and Area

Lesson 10-1

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

A=lw

A=12(8)

8 m

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 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.
- Any irregular figures, just add up all the sides.

Example

18 cm

5 cm

5 cm

P = 2w + 2l

18 cm

P = 2(5) + 2(18)

Perimeter results in regular units, not square units.

P = 10 + 36

P = 46 cm