How can I find the angle between 2 vectors in 3D space. I mean if i am trying to find the angle between up vector (0,1,0) and a vector (1,1,0) using the formula

@ = cos^-1 (a dot b)/ |a|.|b|

then suppose the angle comes to be around 45 but the angle between (0,1,0) and (-1,1,0) also comes to be 45 while it should be -45. So does anybody know of a way through which I can get actual angle between up vector and another vector or is there a way to identify that the 45 angle in the second case is actually -45??? waiting for your reply.... Please help.....!

Fastian