Let the direction cosines of two lines satisfy the equations : 4l + m - n = 0 and 2mn + 10nl + 3lm = 0. Then the cosine of the acute angle between these lines is :
- 107 38
- 10 38
- 103 38
- 203 38
Answer
(3) 103 38
Solution
From the first equation: n = 4l + m
Substitute in the second: 2m(4l + m) + 10l(4l + m) + 3lm = 0
40l2 + 21lm + 2m2 = 0
(8l + m)(5l + 2m) = 0
Case 1: m = -8l n = -4l d.r. (1, -8, -4), magnitude = 9
Case 2: m = - 5l2 n = 3l2 d.r. (2, -5, 3), magnitude = 38
= |(1)(2) + (-8)(-5) + (-4)(3)|9 38
= 309 38 = 103 38
Answer: (3)