# Math Help - Dot Product

1. ## Dot Product

Thank you for all your help in my previous posts! Really appreciate them (:

I have this very simple question but maybe I had erred somewhere, I know how to do it but somehow, I don't get an answer.

The coordinates of the points P,Q,R,S are (4,1,-1), (3,3,5), (1,0,2c) and (1,1,2) respectively.
FInd the value of c so that the vectors OR and PR are orthogonal.

I know this means that the dot product of OR and PR should be 0. For PR, I got (-3,-1, 2c+1) and for OR, (1,0,2c). So I dotted them and ended up with an unsolvable quadratic equation, 4c^2+2c-3=0
Is C supposed to be in surd form using the quadratic formula? Or is there something wrong with my dot product?

2. ## Re: Dot Product

c can assume any value there is no restriction for it. Your dot product appears to be correct to me.