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\newcommand\ds{\displaystyle}
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QUESTION

If $f$ is a function of $x$ and $y$, but $x$ and $y$ both depend
on both $s$ and $t$, write down an expression for
$\ds\frac{\partial f}{\partial s}$ in terms of partial derivatives
of $f$, $x$ and $y$.

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ANSWER

$\ds\frac{\partial f}{\partial s}= \frac{\partial f}{\partial
x}\frac{\partial x}{\partial s}+\frac{\partial f}{\partial
y}\frac{\partial y}{\partial s}$




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