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If vec r is a vector of magnitude 21 ...

If ` vec r` is a vector of magnitude 21 and has direction ratios `2,-3a n d6,` then find ` vec rdot`

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The direction ratios of the `vecr` are 2, -3 and 6.
Therefore, its distance cosines are `l=(2)/(7), m=(-3)/(7) and n=(6)/(7)`. Therefore,
`" "vecr=|vecr|(lhati+mhatj+nhatk)=21((2)/(7)hati-(3)/(7)hatj+(6)/(7)hatk)`
`" "=6hati-9hatj+18hatk`
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