Question 570511
Many different ways. The most common one is

*[tex \LARGE A = \frac{1}{2}bh] (b = base, h = height)


For the next several formulas, denote


a,b,c = side lengths
A,B,C = angles opposite from sides a,b,c
s = semiperimeter (=(a+b+c)/2)
r = inradius
R = circumradius


Other area formulas you'll see are


*[tex \LARGE A = \frac{1}{2}ab\sin C]


*[tex \LARGE A = \sqrt{s(s-a)(s-b)(s-c)}] (Heron's formula)


*[tex \LARGE A = \frac{1}{2}|AB \times AC|] where AB and AC are the vectors from A to B and A to C respectively


*[tex \LARGE A = rs]


*[tex \LARGE A = \frac{abc}{4R}]


*[tex \LARGE A = 2R^2 \sin A \sin B \sin C] (excuse my sloppy notation of using A twice...treat them as different variables)