let f(x,y,z)=2x^2y^2z + 4xy^3z^2 find fxyz(2,1,1)
do i have to find partial dervtive of fx and fy and fz and sub the value in?
I don't think so. You would have to find $\displaystyle f_x$ then partial derivate it with respect to $\displaystyle y$. Once you get this function, partial derivate with respect to $\displaystyle z$.
Then evaluate this function in $\displaystyle (2,1,1)$.
This method however doesn't work for all functions. For example the function $\displaystyle f(x,y)=xy \frac{x^2-y^2}{x^2+y^2}$.