G f(1,1) f(-1,1)Given contour map of z = f(x, y), sketch the gradient vectors (with approximate length and direction): ∇f(1, 1), ∇f(1, −1), ∇f(−1, 1), and ∇f(−1, −1). Describe, how you choose the direction and the length of the gradient vector in each case.