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In the sense where

**AB**//

**CD**,

**AB.CD**=0 right?

However, the examples given in the book only uses the directional vector.

E.g. You are given this line l

_{1}. Then you need to find the foot of the perpendicular from a point P to l

_{1}. So the example use the fact that F (foot of perpendicular) lies on l

_{1}(which l

_{1}vector parametric equation is given) to get the equation of

**OF**and subsequently

**PF**. However, they dot multiplied the

**PF**with the

__direction__vector of l

_{1}and not the equation of l

_{1}which is what I initially thought of.

I know im misunderstanding something but not sure what it is.

Thank you so much for any clarifications given.