# Homework Help: Vector manipulations and stuff

1. Apr 14, 2012

### spaghetti3451

1. The problem statement, all variables and given/known data

Find the angle between the position vectors to the points (3,-4,0) and (-2,1,0) and find the direction cosines of a vector perpendicular to both.

2. Relevant equations

3. The attempt at a solution

You calculate the angle using the two definitions of the dot product (one that uses the magnitude of the vectors and the other that uses the components of the vectors). The answer is 153.4°. Is it right?

The perpendicular vector is the cross product of the two given vectors (the order of the vectors during the operation is immaterial, right?) and it is (0,0,-5) or (0,0,5). Is that right? The direction cosines are found by dividing the each component by the magnitude of the vector, right? So, using (0,0,-5), we get 0,0 and -1 for the direction cosines. Am I right?

I just need to know that my answers are correct, that's all! Please?

2. Apr 14, 2012