Let f(a, y) be a differentiable function which has continuous second order partial derivatives at all points in R2, and such that '( haq + h 20 ' REI + hazs ) = fA where a, b are real constants. Let u = 5 12 13 ' 13 ). Find the value of the directional derivative Du f (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.]