### Note: “ab” means “a” multiplied by “b”. “a^{2}” means “a squared”, which is the same as “a” times “a”.

### Be careful!! Units count. Use the same units for all measurements. Examples

**square** = a^{ 2}

**rectangle** = ab

**parallelogram** = bh

**trapezoid** = h/2 (b_{1} + b_{2})

**circle** = *pi* r^{ 2}

**ellipse** = *pi* r_{1} r_{2}

triangle = |
one half times the base length times the height of the triangle |

equilateral triangle = |

**triangle given SAS (two sides and the opposite angle)
**= (1/2) a b sin C

triangle given a,b,c = [s(s-a)(s-b)(s-c)] when s = (a+b+c)/2 **(Heron’s formula)**

regular polygon = (1/2) n sin(360°/n) S^{2}

when n = # of sides and S = length from center to a corner

Area is measured in “square” units. The area of a figure is the number of squares required to cover it completely, like tiles on a floor.

Area of a square = side times side. Since each side of a square is the same, it can simply be the length of one side squared.

If a square has one side of 4 inches, the area would be 4 inches times 4 inches, or 16 square inches. (Square inches can also be written in^{2}.)

**Be sure to use the same units for all measurements. **You cannot multiply feet times inches, it doesn’t make a square measurement.

The area of a rectangle is the length on the side times the width. If the width is 4 inches and the length is 6 feet, what is the area?

**NOT CORRECT** …. 4 times 6 = 24

**CORRECT**…. 4 inches is the same as 1/3 feet. Area is 1/3 feet times 6 feet = 2 square feet. (or 2 sq. ft., or 2 ft^{2}).