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Question

Show that the points A (−1, 4, −3), B (3, 2, −5), C (−3, 8, −5) and D (−3, 2, 1) are coplanar.

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Solution

The points A, B, C and D will be coplanar iff any one of the following triads of vectors are coplanar: AB, AC, AD; AB, BC, CD; BC, BA, BD, etc.To show that AB, AC, AD are coplanar, we have to prove that their scaler triple product,i.e. AB AC AD = 0Now,AB =3--1 i^ +2-4j^ + -5--3 k^ =4i ^-2 j^ -2 k^ AC =-3--1 i^ + 8-4 j^ + -5--3k^ = -2i^ +4 j^ -2 k^ AD = -3--1 i^ + 2-4 j^ + 1--3 k^ = -2 i^-2 j^ + 4 k^ AB AC AD = 4-2-2-24-2-2-24= 416-4+2-8-4-24+8 = 0Thus, the given points are coplanar.

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