How do you add dipole vectors to determine if a molecule is polar?

[chemistry1]

Hi,

I know what are the steps to determine if a molecule is polar or not, but I'm having one small problem. When I have determined the geometrical shape of my molecule and drew the vectors of the polar bonds that goes from the least electronegative atom to the most electronegative atom, how exactly do I add these vectors? I know that if the sum of my vectors give 0, I have a non polar molecule and if it doesn't then it's polar. But for some easy geometrical shapes I can easily add them, and for some others I don't know. For example, I may know from the beginning that the molecule is non polar because its lewis structure is symmetrical, but when it's time to show it using vectors, for some reason, I'm not able to do it. If I had the sp^3 shape (tetrahedron), and I know that the 4 vectors must give 0, this is where I get stuck.
Anyway, I need help! Thank you again.

 

[Borek]

This is a math problem, not chemistry.

http://en.wikipedia.org/wiki/Euclidean_vector

 

[chemistry1]

Well, I know how to add vectors but only if they're 2d...

 

[DrDu]

Well, you start adding the first and the second, the sum vector lying in the plane spanned by the two vectors. So the problem reduces to the two dimensional case.