Answer
a) See the explanation below.
b) See the explanation below.
Work Step by Step
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, $f_x(a,b)=f_y(a,b)=0$
