I have $\cos^2x\cosh^2y - \sin^2x\sinh^2y$. I saw it written simplified as $\cosh^2 y - \sin^2 x$. But I don't get how to get it.
My attempts were to write $\cosh^2y -1$ instead of $\sinh^2y$ but that way I get $\cos^2x\cosh^2y - \sin^2x\sinh^2y = \cosh^2y(\cos^2x-\sin^2x) + \sin^2x$.
What am I doing wrong?