15. Let f be a function that admits continuous second partial derivatives, for which it is known that: Vf(x,y) = (4x3 - 9ry?, 36y3 - 9x y + 72y? + 36y) fix = 1202 - 972 fyy = 108y? - 9x2 + 144y + 36 y fry = -18ry Consider the critical point (a, b) with positive x-coordinate, such that said critical point generates a local minimum. Determine the value of a: Determine the value of b: 16. If f(x,y) = 23 - y' + xy - 4x + 79 then in the process of obtaining Its critical points the equation is obtained: ay" + by' + cy+d=0 Determine the value of a is: Determine the value of d is: 17. Consider the equation of the surface S given by 2xyz -x"2' + 3y = 6, where a is an odd number and B is an even number. An equation for the normal line to S at the point (-1, -1, 1) is given by: (x, y, z) = (-1, -1, 1) +t(-5, -20,7), tER The value of a is given by: The value of B is given by