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## Homework Statement

I should know this, but I'm using it as a step in one of my proofs and having trouble explaining how it works. Can someone help?

-sin

^{2}x - cos

^{2}x = -1

## Homework Equations

## The Attempt at a Solution

Not sure how to prove and explain it.

And sorry about the title gents I'm using a blackberry so I just copied and pasted the equation from my question : /