The expression that is not a variation of the Pythagorean identity is the third option.
<h3>What is the Pythagorean identity?</h3>The Pythagorean identity can be written as:
For example, if we subtract cos^2(x) on both sides we get the second option:
Which is a variation.
Now, let's divide both sides by cos^2(x).
Notice that the third expression in the options looks like this one, but the one on the right side is positive. The above expression is in did a variation of the Pythagorean identity, then the one written in the options (with the 1 instead of the -1) is incorrect, meaning that it is not a variation of the Pythagorean identity.
Concluding, the correct option is the third one.
If you want to learn more about the Pythagorean identity, you can read: