NCERT Solutions
Class 12 Maths
Three Dimensional Geometry

Ex.11.2 Q.11
Find the angle between the following pairs of lines:
(i)
(ii)
(i) Let b1 and b2 be the vectors parallel to the pair of lines,
So, b1 = 2i + 5j - 3k and b2 = -i + 8j + 4k
Now, |b1| = √{22 + 52 + (-3)2} = √(4 + 25 + 9) = √(38)
And |b2| = √{(-1)2 + 82 + 42} = √(1 + 64 + 16) = √(81) = 9
Now, b1 . b2 = (2i + 5j - 3k).(-i + 8j + 4k)
= -2 * 1 + 5 * 8 - 3 * 4
= -2 + 40 - 12
= 26
The angle, Q, between the given pair of lines is given by the relation,
cos Q =
=> cos Q =
=> Q = cos-1
(ii) Let b1 and b2 be the vectors parallel to the pair of lines,
So, b1 = 2i + 2j + k and b2 = 4i + j + 8k
Now, |b1| = √{22 + 22 + (-1)2} = √(4 + 4 + 1) = √9 = 3
And |b2| = √{42 + 12 + 82} = √(16 + 1 + 64) = √(81) = 9
Now, b1 . b2 = (2i + 2j + k).(4i + j + 8k)
= 2 * 4 + 2 * 1 + 1 * 8
= 8 + 2 + 8
= 16
The angle, Q, between the given pair of lines is given by the relation,
cos Q =
=> cos Q =
=> cos Q =
=> Q = cos-1