Let P( , , ) be the image of the point Q(3, -3, 1) in the line x - 01 = y - 31 = z - 1-1 and R be the point (2, 5, -1). If the area of the triangle PQR is and 2 = 14K, then K is equal to :
- 18
- 81
- 72
- 36
Answer
(2) 81
Solution
[figure]
Foot of perpendicular S(t, t + 3, 1 - t) ⇒ QS = (t - 3, t + 6, -t)
QS (1, 1, -1) ⇒ (t - 3) + (t + 6) + t = 0 t = -1 ⇒ S(-1, 2, 2)
S is the mid-point of PQ ⇒ P(-5, 7, 3)
∵ t = 2 gives (2, 5, -1), R lies on the line ⇒ RS is the height of △PQR on base PQ
QS = 16 + 25 + 1 = 42 ⇒ PQ = 2 42; RS = 9 + 9 + 9 = 3 3
= 12 2 42 3 3 = 9 14 ⇒ 2 = 81 14 = 14K
∴ K = 81
Answer: (2)