# Vectors, dot products and determining the values

1. Jul 21, 2011

### 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

2. Jul 21, 2011

### 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.

3. Jul 21, 2011

### 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 dont know if im on the right track but ill just try and see where it leads

Last edited: Jul 21, 2011
4. Jul 21, 2011

### bngo93

im lost ;/

5. Jul 21, 2011

### micromass

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

6. Jul 21, 2011

### bngo93

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

7. Jul 21, 2011

### 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?

8. Jul 21, 2011

### 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.