Sec2x= select all that apply 1/cos^2x-sin^2x 1/1-2sin^2x 1/sin^2x-cos^2x 1/2sin^2x

The best way to solve such questions is to simplify the options and see what they result in:

Simplifying 1st option:


Simplifying 2nd option:


Simplifying 3rd option:


Simplifying 4th option:


So only first two option are the correct ones as they simplify to sec(2x).

The answer is the numbers between =5 and 5