1.

If `l_(1), m_(1), n_(1), l_(2), m_(2), n_(2) and l_(3), m_(3), n_(3)` are direction cosines of three mutuallyy perpendicular lines then, the value of `|(l_(1),m_(1),n_(1)),(l_(2),m_(2),n_(2)),(l_(3),m_(3),n_(3))|` isA. `l_(3) m_(3) n_(3)`B. `+- 1`C. `l_(1) m_(1) n_(1)`D. `l_(2) m_(2) n_(2)`

Answer» Correct Answer - B
Since, `l_(1), m_(1), n_(1), l_(2), m_(2), n_(2) and l_(3), m_(3), n_(3)` are direction cosines of three mutually perpendicular lines
`:. l_(1)^(2) + m_(1)^(2) + n_(1)^(2) -1, l_(2)^(2) + m_(2)^(2) + n_(2)^(2) = 1`
`l_(3)^(2) + m_(3)^(2) + n_(3)^(2) = 1, l_(1) l_(2) + m_(1) m_(2) + n_(1) n_(2) = 0`
`l_(1) l_(3) + m_(1) m_(3) + n_(1) n_(3) = 0, l_(2) l_(3) + m_(2) m_(3) + n_(2) n_(3) = 0`
Let `Delta = |(l_(1),m_(1),n_(1)),(l_(2),m_(2),n_(2)),(l_(3),m_(3),n_(3))|`. Then,
`Delta^(2) = |(l_(1) ,m_(1),n_(1)),(l_(2),m_(2),n_(2)),(l_(3),m_(3),n_(3))||(l_(1),m_(1),n_(1)),(l_(2),m_(2),n_(3)),(l_(3),m_(3),n_(3))|`
`= |(l_(1)^(2) + m_(1)^(2) + n_(1)^(2),l_(1)l_(2) + m_(1) m_(2) + n_(1) n_(2),l_(1) l_(3) + m_(1) m_(3) + n_(1) n_(3)),(l_(1) l_(2) + m_(1) m_(2) + n_(1) n_(2),l_(1)^(2) + m_(2)^(2) + n_(2)^(2),l_(2) l_(3) + m_(2) m_(3) + n_(2) n_(3)),(l_(1) l_(3) + m_(1) m_(3) + n_(1) n_(3),l_(1) l_(3) + m_(2) m_(3) + n_(2) n_(3),l_(3)^(2) + m_(3)^(2) + n_(3)^(2))|`
`= |(1,0,0),(0,1,0),(0,0,1)| =1`
`:. Delta = +- 1`


Discussion

No Comment Found