# How to find if a vector is parallel to another

1. Sep 19, 2006

### dark_omen

Okay, so I have two vectors a = <-6, 9, -3> and b = <2, -3, 1>. How can I test to see if these two vectors are parallel or not?
Thanks

2. Sep 19, 2006

Vectors a and b are parallel if there exists a real number c such that a=cb. In your case it is pretty obvious.

3. Sep 19, 2006

### dark_omen

What if it is not so obvious like the one I presented. Is there another approach to it (cross product of dot product ??)
Thanks

4. Sep 19, 2006

Think about the definition of the cross product, and see what happens. Btw, just use axb=$$det \left(\begin{array}{ccc}\vec{i} & \vec{j} & \vec{k}\\a_{1} & a_{2} & a_{3}\\b_{1} & b_{2} & b_{3}\end{array}\right)$$ for a=cb.