Let ` vec a ,\ vec b ,\ vec c` be three unit vectors such that `| vec a+ vec b+ vec c|=1\ a n d\ vec a` is perpendicular to ` vec bdot` If ` vec c` makes angle `alpha` and `beta` with ` vec a\ a n d\ vec b` respectively, then `cosalpha+cosbeta=` `-3/2` b. `3/2` c. `1` d. `-1`

Explanation

Consider `vec{a}=i`


`vec{b}=j`

`Vec{c}=-i`

Now
`bar{a}+bar{b}+bar{c}`

`={i}-{i}+{j}`

`={j}`

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