thanks for the helppp
You need to use the chain rule.
$\displaystyle \displaystyle \frac{\partial f}{\partial x}=\frac{\partial f}{\partial s}\frac{\partial s}{\partial x} + \frac{\partial f}{\partial t}\frac{\partial t}{\partial x} $
$\displaystyle \displaystyle \frac{\partial f}{\partial y}=\frac{\partial f}{\partial s}\frac{\partial s}{\partial y} + \frac{\partial f}{\partial t}\frac{\partial t}{\partial y} $
Then just calculate
$\displaystyle \displaystyle xf_x+yf_y=$ and simplify and sub out x's and y's for s's and t's.
i have a question and i make a half of it but it is really hard, i had try so many times but did not get the answer.., please help me to find the answer., sorry for my bad english..IMG.pdf.., only number 1 but ignore du/dx+dv/dy=0.., because i already prove it...