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Given a bot b, |a|=1 and if (a+3b).(2a-b...

Given `a bot b, |a|=1` and if `(a+3b).(2a-b)=-10`, then `|b|` is equal to

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The correct Answer is:
C
`because a bot b`
`:." "a*b=0" ...(i)"`
Also, `(a+3b)*(2a-b)=-10`
`rArr" "2|a|^2-a*b+6b*a-3|b|^2=-10`
` rArr" "2-3|b|^2=-10 rArr |b|=2`
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