# How do you differentiate 4cos x * sin y = 1?

Remember the $y$ is functin of $x$ so:
$4 \left(- \sin \left(x\right) \sin \left(y\right)\right) + 4 \cos \left(x\right) \cos \left(y\right) \frac{\mathrm{dy}}{\mathrm{dx}} = 0$
$\frac{\mathrm{dy}}{\mathrm{dx}} = \frac{\sin \left(x\right) \sin \left(y\right)}{\cos \left(x\right) \cos \left(y\right)} = \tan \left(x\right) \tan \left(y\right)$