1.

Show that thefour points `A ,B , Ca n dD`with positionvectors ` vec a , vec b , vec c`and ` vec d`respectivelyare coplanar if and only if `3 vec a-2 vec b+ vec c-2 vec d=0.`

Answer» Position vectors, `veca,vecb,vecc and vec d` are coplanar if,
`xveca+yvecb+zvecc+wvecd = vec0->(1)`
and
`x+y+z+w = 0`
If we put, ` x = 3, y = -2, z = 1 and w = -2`,
Then, `x+y+z+w = 0`
If we put these values in (1),
`3veca-2vecb+vecc-2vecd = vec0.`
Thus, these `4` vectors are coplanar if
`3veca-2vecb+vecc-2vecd = vec0.`


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