Solve the trig equation..

1-9*cos^2 (2x) = 0

My Work:

1-9*cos^2 (2x)

cos^2 (2x) =1/9

cos (2x) =1/3 or -1/3

2x=arccos(1/3)

x=0.5*arccos(1/3)

or

2x=arccos(-1/3)

x=0.5*arccos(-1/3)

Is this correct? When I plug the value of x into the given trig equation, I do not get 0 = 0.