Question 5 1 pts It is given that the function f(x, y) = -8x3 + 36xy - 9y' - 100 has a local maximum at the point (a, b). Find the value of f(a, b). Question 6 1 pts Let f(x, y) be a differentiable function which has continuous second order partial derivatives at all points in R', and such that Vf = (26xy) + 13y), ax y + bay?), where a, b are real constants. Let u = ( 5 12, 13 ' 13 ). Find the value of the directional derivative Duf(1, 2) of f at the point (1, 2) in the direction u. [Suggestion: Determine a and b first. You may find Clairaut's Theorem useful.]