1. The problem statement, all variables and given/known data This is problem 2(a) from my midterm review sheet, found here: http://www.math.washington.edu/~sullivan/326smt_sp10.pdf 2. Relevant equations Not sure what exactly I need here 3. The attempt at a solution By the quotient rule, ∂f/∂x = [(x2+y2)(y2)-(xy2)(2x)]/[x2+y2]2. Obviously I can't just plug in (0,0). I thought about computing lim(x,y)-->(0,0) f(x,y), but the limit does exist, and doing that seems to be saved for part (b). Is there some theorem I should use to compute ∂f/∂x|(0,0)? Thank you for for help.