Vectors, dot products and determining the values

[bngo93]

vectors a - b + c = 0 ,determine the values of a dot b - adot c - b dot c if

|a|=1, |b|=2 and |c|=3

 

[thepatient]

Have you tried anything so far?

Let's say you have for example vectors w, x, y, if you have x*y + w*y, it is the same as y*(x+w), where * indicates dot product and x+w is the sum of both vectors.

 

[bngo93]

oh i have no idea how to start this problem my first attempt was horrible

i just changed the |a| to a*a by squaring both sides
i don't know if I am on the right track but ill just try and see where it leads
It leads to a dead end : [

 

[bngo93]

im lost ;/

 

[micromass]

Draw -a, b and c on a piece of paper. They form a triangle. Can you say anything special about this triangle??

 

[bngo93]

umm would the values be the ones listed? with the magnitudes
and how did u get -a??

 

[Strants]

I get the feeling micromass meant to say a, -b, and c. I also think he was getting at this: what would the sides of the triangle be?

 

[nickalh]

Yes, I believe micromass meant a, -b, c.
Actually, a triangle is not quite the right word to describe
how the vectors a, -b, c relate to each other.
An improved question:
if we attempt to draw that triangle, what figure do we get?

Hopefully, the answer will be clear, once you have the correct figure. However, if necessary, apply law of cosines
Google can be a good refresher or
http://mathworld.wolfram.com/LawofCosines.html

Please let us know what figure you get, & if you still need help after drawing the figure.