We know that f(x,y) is a function of two variables and if u=ai+bj is a unit vector, then $\displaystyle D_{u}f(x,y)=f_{x}(x,y)a+f_{y}(x,y)b$ is the rate of change of (x,y) at (x,y) in the direction of u. Give a similar formula for the rate of changeof the rate of change off(x,y) at (x,y) in the direction u.