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Question
If `vec"a", vec"b"` are unit vectors and q is the angle between them, show that
`tan theta/2 = |vec"a" - vec"b"|/|vec"a" + vec"b"|`
Solution
We have `sin theta/2 = 1/2|vec"a" - vec"b"|`
`cos theta/2 = 1/2|vec"a" + vec"b"|`
`(sin theta/2)/(cos theta/2) = (1/2|vec"a" - vec"b"|)/(1/2|vec"a" + vec"b"|)`
`tan theta/2 = |vec"a" - vec"b"|/|vec"a" + vec"b"|`
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