what does cos2x mean?

$$cos(2x) \\ = cos^2x-sin^2x\\ =1-sin^2x-sin^2x\\ =1-2sin^2x$$

Cos(2x) is just a "double angle" trig function......there's nothing to "do" on a calculator, unless we were trying to evaluate the cosine for some value of 2x where x is an angle and 2x is twice the measure of that angle.......