We know that if (x, y) is a function of two variables and if u = ai

We know that if ƒ(x, y) is a function of two variables and if u = ai + bj is a unit vector, then D_u ƒ(x, y) = ƒ_x(x, y)a + ƒ_y(x, y)b is the rate of change of ƒ(x, y) at (x, y) in the direction of u. Give a similar formula for the rate of change of the rate of change of ƒ(x, y) at (x, y) in the direction u.