Find the directional derivative of f at the given point in the direction
indicated by the angle theta.
a) f(x, y) = (x^2 − y)^3, (3, 1), theta = 3pi/4.
b) f(x, y) = sin(x + 2y), (4,−2), theta = −2pi/3.
now, our problem here is finding the unit vector. if you are completely lost, just draw a diagram with the unit circle to figure out the vector that points in the direction you want. recall, therefore, that unit vectors in the direction are given by