Perimeters and Areas of Rectangles

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Perimeters and Areas of Rectangles

• Lesson 9-3

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Area

• Area is the amount of space inside a figure. We
  use square units to describe area.
• For rectangles, the area formula is Alw (area
  equals length times width).
• For squares, the formula is still Alw.
  However, since every side of a square is the
  same, then it can also be expressed as As2 where
  s equals the length of one side.

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Example
12 m
Alw
A12(8)
8 m
A96 sq. m
Area is expressed in square units. It can be
written (in this case) as 96 sq. m or 96 m2.
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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.

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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.

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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
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Homework Time