Question

The vectors which is/are coplanar with vectors `hati+hatj+2hatk and hati+2hatj+hatk` and perpendicular to the vector `hati+hatj+hatk ` is /are (A) `hatj-hatk` (B) `-hati+hatj` (C) `hati-hatj` (D) `-hatj+hatk`
A. `hatj - hatk`
B. `-hati + hatj`
C. `hati -hatj`
D. `-hatj +hatk`

Answer 1

Correct Answer - a,d
Any vector in the plane of `veca = hati +hatj + 2hatk and vecb = hati + 2hatj + hatk` is
`vecr=lamda(hati+hatj+2hatk) +mu(hati+2hatj+hatk)`
`= (lamda+mu)hati+(lamda+2mu)hatj= (2lamda+mu)hatk`
Also `vecr` is perpendicular to the vector `hati + hatj +hatk`
`Rightarrow vecr.vecc=0`
` Rightarrow lamda + mu =0`
Possible vectors are `hatj -hatk or -hatj +hatk`