Question

@Michele_Laino

Answer 1

What is the question?

Answer 2

youhave to apply the derivative of the composed function, better is rewrite your function as below:
\[f(x)=\frac{ 1 }{ \cos(2x) }\]

Answer 3

so:
\[f'(x)=-\frac{ f(x) }{ [f(x)]^{2} }\]

Answer 4

please try to complete the calculus, note that you have to peform the derivative of the functiony=2x

Answer 5

I think the third answer