1. The problem statement, all variables and given/known data If f(x,y) = x(x^2+y^2)^(-3/2)*e^(sin(x^2y)) find the derivative of f with respect to x at the point (1,0). 3. The attempt at a solution The textbook solution just plugs 0 into y and gets f(x) = x^-2 and then proceeds to differentiate this resulting in the answer -2. I don't understand why this is legal. How can you just plug the point into the function and then take the derivative? For example, if I had the function y = x and I wanted the derivative at x=0. You can't just plug in zero and take the derivative...?