00:01
Ok, so in this exercise we have our function f of x, y, z equal to x squared y minus y z cubed plus z.
00:13
Ok, part a of our exercise.
00:15
Let's compute the gradient of f.
00:17
Well, the gradient of f is going to be the partial derivative of f with respect to x, which is 2xy, the partial derivative with respect to y, which is x squared minus z cubed, the partial derivative with respect to z, which is 1 minus 3y z squared.
00:36
Ok, now part b of our exercise.
00:38
Let's evaluate the gradient at the point p equal to 1, negative 2, 0.
00:48
Ok, well, the evaluation here is going to be negative 4, 1, 1.
00:59
Perfect.
01:00
Now, part c of our exercise, the directional derivative in the direction of the unit vector u.
01:08
Well, the unit vector u is 1 over the length of b multiplied by v.
01:15
So we have 1 over square root of 4 plus 1 plus 4, which is 9.
01:23
And this one is just the square root of 9, which is 3.
01:27
So 1 third multiplied by v, which is 2, 1, negative 2.
01:34
Perfect.
01:35
Now, the directional derivative evaluated at p...