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Question:
Three vertices of a parallelogram ABCD are A(3, - 1, 2), B (1, 2, - 4) and C (- 1, 1, 2). Find the coordinates of the fourth vertex.
Answer:

Let fourth vertex is D (x,y,z)

Now mid point of AC = {(3-1)/2, (-1+1)/2, (2+2)/2} = (1,0,2)

      mid point of BD = {(x+1)/2, (y+2)/2, (z-4)/2}

Now 

      mid point of AC = mid point of BD

=> (1,0,2) = {(x+1)/2, (y+2)/2, (z-4)/2}

    (x+1)/2 = 1

=> x+1 = 2

=> x = 2-1

=> x = 1

(y+2)/2 = 0

=> y+2 = 0

=> y = -2

(z-4)/2 = 2

=> z-4 = 2*2

=> z-4 = 4

=> z = 4+4

=> z = 8

So the point D is (1,-2,8) 

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