The cosine rule is a formula you would use for math. You would use it to find the side length of a triangle.
It can be written as a^2 = b^2 + c^2 (i think :| )
Sohcahtoa:
Sohcahtoa is a acronym it has to do with math, along the lines of
Sine -- Soh
Cosine -- Cha
Tangent -- Toa
Soh = Sine equals Opposite over Hypotenuses
Cha = Cosine equals Adjacent over Hypotenuses
Toa = Tangent equal Oppsoite over Adjacent
H0P3 It H3LPS :)