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Prove that |vecPxxvecQ|^2=|vecP|^2|vecQ|...

Prove that `|vecPxxvecQ|^2=|vecP|^2|vecQ|^2-|vecP*vecQ|^2`.

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`|vecPxxvecQ|^2+|vecP*vecQ|^2=(PQsin theta)^2 +(PQ cos theta)^2`
`[theta ="angle from "vec P "to" vecQ]`
`=P^2Q^2(sin^2theta+cos^2theta)=P^2Q^2`
`=|vecP|^2|vecQ|^2`
`therefore |vecPxxvecQ|^2=|vecP|^2|vecQ|^2-|vecP*vecQ|^2`.
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