Certainly the formula encountered earliest by most students is $A=\frac12 bh$, where $b$ is a a side length (called a base), and $h$ is the length of an altitude/height. Some other useful methods are: Using the [[in-radius]] $r$ and the [[perimeter]] $P$: $A=\frac{rP}{2}$ ![[in-radius-and-perimeter-formula.svg]]