1.

Let `veca=(1,1,-1), vecb=(5,-3,-3)` and `vecc=(3,-1,2)`. If `vecr` is collinear with `vecc` and has length `(|veca+vecb|)/(2)`, then `vecr` equalsA. `+-3vecc`B. `+-3/2vecc`C. `+-vecc`D. `+-2/3vecc`

Answer» Correct Answer - C
We have, `vecr=lambdavecc`
Given, `|vecr|=|lambda||vecr|`
`therefore |6hati-2hatj-hatk|=2|lambda||3hati-hatj+2hatk|`
`therefore sqrt(56) = 2|lambda|sqrt(14)`
`therefore lambda=+-1`
`therefore vecr=+-vecc`


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