EXAMPLE 4 Find the directional derivative of the function f(x, y) = x2y3 3y at the point (2, 1) in the direction of the vector v = 5i + 2j. SOLUTION First we compute the gradient vector at (2, 1). Note that v is not a unit vector, but since |v| = |:| , the unit vector in the direction of v is v u = = . |v| |:| Therefore, by this theorem, we have Duf(2: -1) = Vf(2, -1) - u (+E)-(E+n 4- +9-2 \\/2_9 E