Partial derivatives allow us to approximate functions just like ordinary derivatives do, only with a contribution from each variable. If x≈a and y≈b, then
f(x,y)≈f(a,b)+(fx(a,b))(x−a)+(fy(a,b))(y−b).
This is sometimes written as df=∂f∂xdx+∂f∂ydy.
And, just as in one dimension, we can use higher derivatives to get a more accurate approximation.
Taylor Series in two variables:f(x,y)=∞∑n,m=0cn,m(x−a)n(y−b)m,