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Question
If `veca` is a unit vector perpendicular to `vecb` and `(veca + 2vecb).(3veca - vecb) = -5`, find `|vecb|`.
Solution
As per question
`|veca| = 1`, `veca.vecb = 0`
∴ `(veca + 2vecb).(3veca - vecb) = -5`
`\implies 3|veca|^2 + 5(veca.vecb) - 2|vecb|^2 = -5`
`\implies 2|vecb|^2 = 8`
`\implies |vecb| = 2`
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