Heron's Formula
Heron's formula is just a nifty formula that's handy if you have three sides of a triangle and don't want to apply the law of cosines/sines to find the angles.
To use it, you take half the perimeter, = "s", and...
sqrt[s(s-a)(s-b)(s-c)]
...with a, b, c being equal to any of the three sides (it doesn't matter which, just be consistent.)
I figured it'd be important to point out that these values would give you four distances multiplied together, not three. When you take the square root of these multiplied distances, you get (some distance)^2.
for example, if you were to use inches...
inches*inches*inches*inches = sqrt(in^4) = in^2
Both methods are useful in the right situation