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Old Sep 27th 2009, 05:35 PM   #1
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angle..

Two vectors A and B have precisely equal magnitudes. For the magnitude of A + B to be 68 times greater than the magnitude of A - B, what must be the angle between them?

i ended up to cos-1(4623/4625) but it says wrong answer.
help me out please
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Old Sep 29th 2009, 04:29 AM   #2
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Try the cosine rule of vector addition

$\displaystyle
c^2\ =\ a^2\ +\ b^2\ -\ 2\ a\ b\ cos\ \theta
$

where $\displaystyle \theta$ is the angle between them.

For a - b , use a + (-b)
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