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Partial derivative   (English)

Definition (advanced level)

If f(x, y) is a function of both x and y, then the partial derivative of f with respect to x, written [(∂f)/(∂x)], is:
f

x
=
lim
h → 0 
f(x + h, y)f(x, y)

h

This is equivalent to differentiating with respect to x and treating y as though it were a constant. It can also be written fx.
For example, if f(x, y) = x2 + xy + 2 y2,
then [(∂f)/(∂x)] = 2 x + y
We can define partial derivatives with respect to z, and of functions of three or more variables, in the same way.

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