From the 2nd to 3rd step, if you divided by cos x you have to divide all terms of the numerator however you only divided the (cos x)^2.


Actually if you did the division throughout, you would have ended back the at first step.

The solution I provided continues from your 2nd step and I made use of the identity (sinx)^2 + (cosx)^2 = 1.

Cheers!