Let the line L pass through the point (-3, 5, 2) and make equal angles with the positive coordinate axes. If the distance of L from the point (-2, r, 1) is 143, then the sum of all possible values of r is :
- 16
- 12
- 6
- 10
Answer
(4) 10
Solution
Equal angles with positive axes ⇒ direction ratios of L: (1, 1, 1)
Let A(-3, 5, 2) on L and Q(-2, r, 1)
AQ = (1, r - 5, -1)
Projection on L = 1 + (r - 5) - 1 3 = r - 5 3
Distance2 = | AQ|2 - (projection)2
= 2 + (r - 5)2 - (r - 5)23 = 2 + 2(r - 5)23
2 + 2(r - 5)23 = 143
(r - 5)2 = 4 ⇒ r = 7 or r = 3
Sum = 7 + 3 = 10
Answer: (4)