Physics Help Forum angle..

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Sep 27th 2009, 05:35 PM #1 Senior Member   Join Date: Sep 2009 Location: St. John's, NL Posts: 124 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
 Sep 29th 2009, 04:29 AM #2 Physics Team   Join Date: Feb 2009 Posts: 1,425 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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