NCERT Solutions
Class 12 Maths
Three Dimensional Geometry

Ex.11.1 Q.5
Find the direction cosines of the sides of the triangle whose vertices are (3, 5, − 4), (−1, 1, 2) and (− 5, − 5, − 2).
The vertices of ∆ABC are A (3, 5, −4), B (−1, 1, 2), and C (−5, −5, −2).
The direction ratios of side AB are (−1 − 3), (1 − 5), and (2 − (−4)) i.e., −4, −4, and 6.
Then, √{(-4)2 + (-4)2 + (6)2} = √(16 + 16 + 36) = √(68) = 2√(17)
Therefore, the direction cosines of AB are
, -
,
=
=
The direction ratios of BC are (−5 − (−1)), (−5 − 1), and (−2 − 2) i.e., −4, −6, and −4.
Therefore, the direction cosines of BC are
, -
,
=
=
The direction ratios of CA are (−5 − 3), (−5 − 5), and (−2 − (−4)) i.e., −8, −10, and 2.
Therefore, the direction cosines of AC are
,
,
=
=
=
=