Question: if cos x = 2 then what is cos2x?
Well, first of all, you can't have cos(x) = 2, since cos(x) is a sinusoidal function that varies in size between -1 and +1 only.
Taking cos(x) = 0.2.
There is a double-angle formula,
cos(2A) = cos^2(A) - sin^2(A) = 1 - 2sin^2(A) = 2cos^2(A) - 1.
Using cos(2x) = 2cos^2(x) - 1, with cos(x) = 0.2, then
cos(2x) = 2(0.2)^2 - 1 = 0.08 - 1 = -0.92
cos(2x) = -0.92