The number of solutions to the equation x = 5cos2x where x is a subset of the R is?

If tan(x) = 1/a

sin^2(x) = 1/b

cos(x) = ?

I tried using the identities but I cant seem to get right answer.

The intensity, I, of a light bumb is I = k/(x^2) where x = distance from bulb. THe rate at which the intensity changes with respect to distance when x = 5 is?

THis one is dI/dx right? So I get -2k/5, which is wrong.