## Section 14.6 Problem 5

$f(x,y)=\cos(x)-\cos(x)+\sin(y)-\sin(y)$

by differentiating the trig functions, but that would be silly. If you want more of a hint, recall that

$u^2\cos^2(v)+u^2\sin^2(v)=u^2.$

So yes, for many of you (depending on the randomness of the functions given to you), the derivative with respect to one of the variables is actually a number.