1.

The points with position vectors `alpha hati+hatj+hatk, hati-hatj-hatk, hati+2hatj-hatk, hati+hatj+betahatk` are coplanar ifA. `(1-alpha)(1+beta)=0`B. `(1-alpha)(1-beta)=0`C. `(1+alpha)(1+beta)=0`D. `(1+alpha)(1-beta)=0`

Answer» Correct Answer - A
Let `P,Q,R` and `S` be the given points with position vectors `alpha hati+hatj+hatk, hati-hatj-hatk, hati+2hatj-hatk` and `hati+hatj+betahatk` respectively. Then,
`vec(QP)=(alpha-1)hati+2hatj+2hatk, vec(QR)=0hati+3hatj+0hatk`
and `vec(QS)=0hati+2hatj+(beta+1)hatk` are coplanar.
`:.[vec(QP),vec(QR),vec(QS)]=0`
`implies|(alpha-1,2,2),(0,3,0),(0,2,beta+1)|=0`
`implies (alpha-1)(beta+1)=0implies(1-alpha)(1+beta)=0`


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