sin(xy)+z^2=sin(x+z)
ycos(xy)+2z (δz/δx) = [ 1+(δz/δx) ] cos(x+z)
[2z - cos(x+z)](δz/δx) = cos(x+z) - ycos(xy)
δz/δx = [cos(x+z) - ycos(xy)]/[2z - cos(x+z)]