I've been at this problem for a while!
Find the directional derivative of the given function at the point (3,-1) in the direction theta=(pi)/4.
f(x,y)=4x+xy^2-5y
fx=4+y^2
fy=2xy-5
applying ponts:
fx=4+(-1)^2=5
fy=2(3)(-1)-5=-11
<5, -11>
But how would I find my unit vector in the direction of theta=pi/4?