Formulae
Trigonometric Multiples
sin(3x) = 3·sin(x) - 4·sin3(x)
cos(3x) = 4·cos3(x) - 3·cos(x)
sin(5x) = 5·sin(x) - 20·sin3(x) + 16·sin5(x)
Law of Sines
a/sin(A) = b/sin(B) = c/sin(C)
Law of Cosines
c2 = a2 + b2 - 2ab·cos(C)
Taylor Series
sin(x) = x - x3/3! + x5/5! - x7/7! + x9/9! - x11/11! + ...
Obviously, there are a lot more formulae, but these are what I have needed recently. --Richard Martino