Hi I am stuck on this problem. Can someone please explain. Thanks!
Let Compute
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Here's a hint: by the chain rule
$\displaystyle \frac {\partial w}{\partial s} = \frac {\partial w}{\partial x} \cdot \frac {\partial x}{\partial s} + \frac {\partial w}{\partial y} \cdot \frac {\partial y}{\partial s} + \frac {\partial w}{\partial z} \cdot \frac {\partial z}{\partial s}$
and, similarly,
$\displaystyle \frac {\partial w}{\partial t} = \frac {\partial w}{\partial x} \cdot \frac {\partial x}{\partial t} + \frac {\partial w}{\partial y} \cdot \frac {\partial y}{\partial t} + \frac {\partial w}{\partial z} \cdot \frac {\partial z}{\partial t}$
(2,-1) refers to (s,t) it seems (that is, if the problem described the functions as x(s,t) for instance). you can find the corresponding x, y and z values for this point