NCERT Solutions
Class 12 Maths
Three Dimensional Geometry

Ex.11.3 Q.1
In each of the following cases, determine the direction cosines of the normal to the plane and the distance from the origin.
(a) z = 2
(b) x + y + z = 1
(c) 2x + 3y – z = 5
(d) 5y + 8 = 0
(a) The equation of the plane is z = 2 or 0.x + 0.y + z = 2 ………..1
The direction ratios of normal are 0, 0, and 1.
So, √(02 + 02 + 12) = 1
Dividing both sides of equation 1 by 1, we get
0.x + 0.y + 1.z = 2
This is of the form lx + my + nz = d, where l, m, n are the direction cosines of
normal to the plane and d is the distance of the perpendicular drawn from the origin.
Therefore, the direction cosines are 0, 0, and 1 and the distance of the plane from the origin is 2 units.
(b) Given, x + y + z = 1 ………1
The direction ratios of normal are 1, 1, and 1.
So, √(12 + 12 + 12) = √(1 + 1 + 1) = √3
Dividing both sides of equation 1 by √3, we get
……….2
This equation is of the form lx + my + nz = d, where l, m, n are the
direction cosines of normal to the plane and d is the distance of normal from the origin.
Therefore, the direction cosines of the normal are ,
and
and the distance of normal from the origin is
units.
(c) Given, 2x + 3y − z = 5 ………1
The direction ratios of normal are 2, 3, and −1.
So, √{22 + 32 + (-1)2} = √(4 + 9 + 1) = √14
Dividing both sides of equation 1 by √14, we get
…….2
This equation is of the form lx + my + nz = d, where l, m, n are the direction cosines
of normal to the plane and d is the distance of normal from the origin.
Therefore, the direction cosines of the normal to the plane are ,
and
and the distance of normal from the origin is units.
(d) Given, 5y + 8 = 0
0x − 5y + 0z = 8 ........... (1)
The direction ratios of normal are 0, −5, and 0.
So, √{02 + (-5)2 + 02} = 5
Dividing both sides of equation 1 by 5, we get
-y = …….2
This equation is of the form lx + my + nz = d, where l, m, n are
he direction cosines of normal to the plane and d is the distance of normal from the origin.
Therefore, the direction cosines of the normal to the plane are 0, −1, and 0 and the distance of normal from the origin is units.