# Math help.. Algebra and Geometry...

Discussion in 'After Hours Lounge (Off Topic)' started by Andrew S, Dec 13, 2003.

1. ### Andrew S Stunt Coordinator

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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

2. ### Garrett Lundy Producer

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My head hurts. I go sleepy now.

3. ### Andrew S Stunt Coordinator

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lol.. turns out unit vector means I just make it equal 1. duh...

4. ### Dome Vongvises Lead Actor

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Vectors, as you'll learn in physics, are quantities that possess both magnitude and direction.

5. ### Andrew Testa Second Unit

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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

6. ### george kaplan Executive Producer

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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.

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8. ### Will Pomeroy Stunt Coordinator

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