4. Using Second Derivative Test (or other method) to find local minimum or/and maximum value of f(x, y) = 1 + 6x - 8y + x2 + y2. (Note: D(x, y) = faxfun - (fav)2) (a) what is the critical point(s)? (b) does this function have minimum value? If so what is the minimum value? (c) does this function have maximum value? If so what is the maximum value? (d) what is the saddle point of it?7. Find the mass of the lamina bounded by the equations 0