Consider the function $z(x, y),$ its partial derivatives $(\partial z / \partial x)_{y}$ and $(\partial z / \partial y)_{x},$ and the total derivative $d z / d x$
(a) How do the magnitudes $(\partial x)_{y}$ and $d x$ compare?
(b) How do the magnitudes $(\partial z)_{y}$ and $d z$ compare?
(c) Is there any relation among $d z$ ( $\partial z_{x},$ and $(\partial z)_{y} ?$