Let a triangle PQR be such that P and Q lie on the line x+38= y-42= z+12 and are at a distance of 6 units from R(1, 2, 3). If ( , , ) is the centroid of PQR, then + + is equal to :
- 4
- 5
- 6
- 8
Answer
(3) 6
Solution
Any point on the line is (8t - 3, 2t + 4, 2t - 1). Its distance from R(1, 2, 3) is 6, so
(8t - 4)2 + (2t + 2)2 + (2t - 4)2 = 36
72t2 - 72t + 36 = 36 72t(t - 1) = 0 t = 0 or 1
which gives P(-3, 4, -1) and Q(5, 6, 1).
Centroid ( , , ) = ( -3 + 5 + 13, 4 + 6 + 23, -1 + 1 + 33 ) = (1, 4, 1)
+ + = 6
Answer: (3)