Step 1:

The function is , point at and angle is .

Consider .

Apply partial derivative on each side with respect to x.

Substitute in the .

Apply partial derivative on each side with respect to y.

Substitute in the .

Step 2:

The directional derivative in the direction of a unit vector then

.

The directional derivative of the function at a point in the direction is

.

Solution :

The directional derivative of the function is .