Ma error with cos-1(√3/2)?

Question

I am trying to answer sec2x = 2 / √3

I did
1/cos2x = 2/√3
√3/2 = cos2x

Then I put cos-1(√3/2) in my calculator and got a math error, what am I forgetting?

Answer 1

maybe you have to push 2ndF or something alike to get Arc cos x
Normally cos x and cos^-1 x are at the same place
2x = pi/6 so x = pi/12