NCERT Solutions
Class 12 Maths
Three Dimensional Geometry

Ex.11.2 Q.3
Show that the line through the points (4, 7, 8) (2, 3, 4) is parallel to the line through
the points (−1, −2, 1), (1, 2, 5).
Let AB be the line through the points, (4, 7, 8) and (2, 3, 4), and CD be the line through the points, (−1, −2, 1) and (1, 2, 5).
The directions ratios, a1, b1, c1 of AB are (2 − 4), (3 − 7), and (4 − 8) i.e., −2, −4, and −4.
The direction ratios, a2, b2, c2 of CD are (1 − (−1)), (2 − (−2)), and (5 − 1) i.e., 2, 4, and 4.
AB will be parallel to CD, if =
=
=
= -1
=
= -1
=
= -1
So, =
=
Thus, AB is parallel to CD.