Video Example () EXAMPLE 4 Find the directional derivative of the function f(x, y) = x2y - 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). Vf ( x, y ) = + (3x212 - 3)j Vf(2, -1) = -4i + Note that v is not a unit vector, but since |v| = the unit vector in the direction of v is u = . IVI Therefore, by this theorem, we have Duf(2, -1) = VF(2, -1) . u = (-41 + 2 + V29 j -4 . + 9 . 2 V 29