NCERT Solutions
Class 12 Maths
Three Dimensional Geometry

Ex.Misc.Q.2
If l1, m1, n1 and l2, m2, n2 are the direction cosines of two mutually perpendicular lines,
show that the direction cosines of the line perpendicular to both of these are m1n2 − m2n1, n1l2 − n2l1, l1m2 − l2m1.
It is given that l1, m1, n1 and l2, m2, n2 are the direction cosines of two mutually
perpendicular lines.
Let l, m, n be the direction cosine of the line perpendicular to each of the given line.
Therefore,
ll1 + mm1 + nn1 = 0 ……….1
ll2 + mm2 + nn2 = 0 ……….2
Solving equation 1 and 2, we get
Thus, the direction cosines of the given line are proportional to
(m1n2 – m2n1), (n1l2 – n2l1), (l1m2 – l2m1)
So, its direction cosines are
Where k = √[(m1n2 – m2n1)2 + (n1l2 – n2l1)2 + (l1m2 – l2m1)2]
We know that (l12 + m12 + n12) (l22 + m22 + n22) – (l1l2 + m1m2 + n1n2)2
= (m1n2 – m2n1)2 + (n1l2 – n2l1)2 + (l1m2 – l2m1)2 …………3
It is given that the given lines are perpendicular to each other.
So, l1l2 + m1m2 + n1n2 = 0
Also, we have
l12 + m12 + n12 = 1 and l22 + m22 + n22 = 1
From equation 3, we get
(m1n2 – m2n1)2 + (n1l2 – n2l1)2 + (l1m2 – l2m1)2 = 1
So, k = 1
Hence, the direction cosines of the given line are (m1n2 – m2n1), (n1l2 – n2l1), (l1m2 – l2m1).