The triple product of coplanar vectors equals zero.
The triple product equals
Thus, the coplanarity condition is
Example 2. Show that the four points А (1;2;–1), В (0;1;5), С (–1;2;1), and D (2;1;3) belong to the same plane.
To show that they are coplanar, we find the triple product
Thus, the four points belong to the same plane.
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, because 
.
B
C
A
D
={–1;–1;6},
={–2;0;2},
={1;–1;4}.
.
