## Calculus 8th Edition

a) This means that when the first-order partial derivative of $f(x,y)$ exists then $f_x(a,b)=f_y(a,b)=0$ b) The critical point of $f(x,y)$ signifies that a point on $(a,b)$ when the first-order partial derivative of $f(x,y)$ does not exist that is, $f_x(a,b)=f_y(a,b)=0$