Find for which the points A(3, 2, 1), B(4, λ, 5), C(4, 2, – 2) and D(6, 5, – 1) are coplanar.
Formula: -
and 
then,
let position vector of
position vector of
position vector of
position vector of
The four points are coplanar if the vector 
are coplanar
now, using
⇒ 1(9) – (λ – 2)( – 2 + 9) + 4(3 – 0) = 0
⇒ 7λ = 35
⇒ λ = 5
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