1. Problem: if f(1,3)=7, use Taylor expansion to describe f(1.2,3.1) and f(.9,2.8) if the partials of f are give by df/dx=.2 d^2f/dx^2=.6 df/dy=.4 d^2f/dy^2=.9 (you do not need to go beyond the second derivative for this problem) 2. I know from class how to do this if one variable changes. I thought I knew how to do it with 2, but I am getting the wrong answer. if I wanted to do f(1.2,3) I would do f(1,3)+df/dx*(.2)+d2f/dx2*(.2)^2=7+(.2)*(.2)+(.2)^2/2*(.6). so I thought if I wanted to do both, I would expand in both variables: f(1.2,3.1) ~ f(1,3)+df/dx*(.2)+d2f/dx2*(.2)^2+df/dy*(.1)+d2f/dy2*(.1)^2 Why is this coming out to be the wrong answer?