How is direction vector calculated????!!!..Please explain?

Question

We are doing direction derivatives in class and our lecturer is not really explaining much at all. I can't seem to understand how the direction vector is found at all. There is this question where the directional derivative is to be found at 1 point in the direction of another given point. I keep getting the direction vector wrong. Our lecturer didn't really clear up how the direction vector actually works and I still am not clear about it. It would be great if anyone can explain to me how the direction vector for let's say the following question is found:
Find the directional derivative of f(x,y,z)=y/(x+z) at P(2,1,-1) in the direction from P to Q(-1,2,0).
PS: I am not really sure if the direction vector is (2,1,-1)-(-1,2,0)=(3,-1,-1) or (-1,2,0)-(2,1,-1)=(-3,1,1)...HELP PLEASE!!!!

Answer 1

Good textbook is better than bad teacher.
You may find something on the Internet too:
http://www.dogpile.com/dogpile/ws/results/Web/directional%20derivative/1/417/TopNavigation/Relevance/zoom=off/_iceUrlFlag=7?_IceUrl=true
Simple explanation is here:
http://mathworld.wolfram.com/DirectionalDerivative.html
The direction vector from P(2,1,-1) to Q(-1,2,0) is (Q-P)= (-3,1,1).
For solution you will need also absolute value of the direction vector (√11) and partial derivatives of the function.
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Answer 2

At any on the spot of time: No. At any given time Centripetal rigidity is in the direction of the middle of rotation Over a quantity of time: confident. Centripetal rigidity rotates, the path is continuously changing.