Thus, C(x,y) has one critical point at (6,8).

To determine the type, we will use the second derivative test by finding all second derivatives at the point (6,8).

Recall that

P_x (x,y)=-2x+4y-20 P_y (x,y)=4x-14y+88

1- find all second derivatives, why do we want to know the second derivative ?

2- Why do we choose these specific numbers and not others?