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Question
Show that `(veca - vecb) xx (veca + vecb) = 2(veca xx vecb)`.
Solution
We have to prove that, `(veca + vecb) xx (veca - vecb) = 2(veca xx vecb)`
`(veca + vecb) xx (veca - vecb) = (veca - vecb) xx veca + (veca - vecb) xx vecb`
= `veca xx veca - vecb xx veca + veca xx vecb - vecb xx vecb`
= `2(veca xx vecb)`
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