Area
The area of a plane figure, measured in square units (or units ^{ 2 } ), is the number of squares required to cover it completely .
Example:
The figure shown has an area of $9.5$ square units: $8$ whole squares plus $3$ halfsquares.
Here are some formulas for the areas of common plane figures.


Shape

Formula

Variables

Square


$s$ is the length of the side of the square. 
Rectangle


$L$ and $W$ are the lengths of the rectangle's sides (length and width). 
Triangle

$A=\frac{1}{2}bh$

$b$ and $h$ are the base and height . 
Triangle

$\begin{array}{l}A=\sqrt{s\left(sa\right)\left(sb\right)\left(sc\right)}\\ \text{Where}\text{\hspace{0.17em}}\text{\hspace{0.17em}}s=\frac{a+b+c}{2}\end{array}$

$a$ , $b$ , and $c$ are the side lengths and $s$ is the semiperimeter 
Parallelogram


$b$ is the length of the base and $h$ is the height. 
Trapezoid

$A=\frac{{b}_{1}+{b}_{2}}{2}h$

${b}_{1}$ and ${b}_{2}$ are the lengths of the parallel sides and $h$ the distance (height) between the parallels. 
Circle

$A=\pi {r}^{2}$

$r$ is the radius. 