I'm taking an Algebra and Geometry class through correspondance and I'm having some troubles with a couple questions... 52. If vector a and vector b are unit vectors and the angle (theta) between them is 120 degrees, calculate (5a-b)*(a+b) The second question is pretty much the same setup... I know that I have to use the equation: cos theta = a * b / |a||b| I just get lost interpretting the significance of a and b being unit vectors? Any tips would be greatly appreciated Thanks, Andrew

Andrew, You pretty much got it. a unit vector is a vector with a length (magnitude) of 1, regardless of its direction. So if the question was asking you to use the vector length in the calculation, just use 1, and the angle was a red herring. Andy

Well something's funky. If cos theta = a * b / |a||b| then cos(theta)=1*1/1*1=1 but cos(120)=.8142 so it would appear that the a & b don't represent length of the vector, since it's certainly possible to have two unit vectors with an angle of 120 degrees. I don't know what a & b are supposed to be in that formula for the cos (theta), but it isn't length.

Careful about helping. There are many final exams occuring right now. Both in traditional classes and via correspondance courses. Hate to cause a problem, but be careful. Dr. Joe