Let a = i + 2 j + k and b = 2 i + 7 j + 3 k. Let L1 : r = (- i + 2 j + k) + a, R and L2 : r = ( j + k) + b, R be two lines. If the line L3 passes through the point of intersection of L1 and L2 and is parallel to a + b, then L3 passes through the point :
- (-1, -1, 1)
- (2, 8, 5)
- (8, 26, 12)
- (5, 17, 4)
Answer
(3) (8, 26, 12)
Solution
A general point of L1 is (-1 + , 2 + 2 , 1 + ) and of L2 is (2 , 1 + 7 , 1 + 3 ).
At the point of intersection,
1 + = 1 + 3 = 3
-1 + = 2 -1 + 3 = 2 = 1, = 3
Check y : 2 + 2(3) = 8 = 1 + 7(1) ✓
so the point of intersection is (2, 8, 4).
With a + b = 3 i + 9 j + 4 k as direction,
L3 : x - 23 = y - 89 = z - 44 = t
and its general point is (2 + 3t, 8 + 9t, 4 + 4t).
For t = 2 this is (8, 26, 12), which lies on L3.
(For the other options no single value of t satisfies all three coordinates.)
Answer: (3)