NCERT Solutions
Class 12 Maths
Three Dimensional Geometry

Ex.11.3 Q.13
In the following cases, determine whether the given planes are parallel or perpendicular, and in case they are neither, find the angles between them.
(a) 7x + 5y + 6z + 30 = 0 and 3x – y – 10z + 4 = 0
(b) 2x + y + 3z - 2 = 0 and x – 2y + 5 = 0
(c) 2x - 2y + 4z + 5 = 0 and 3x – 3y + 6z - 1 = 0
(d) 2x - y + 3z - 1 = 0 and 2x – y + 3z + 3 = 0
(e) 4x + 8y + z - 8 = 0 and y + z - 4 = 0
The direction ratios of normal to the plane L1: a1x + b1y + c1z = 0 are a1, b1, c1 and L2: a2x + b2y + c2z = 0 are a2, b2, c2
L1 || L2 if =
=
L1 Ʇ L2 if a1a2 + b1b2 + c1c2 = 0
The angle between L1 and L2 is given by,
Q = cos-1
(a) The equations of the planes are 2x + y + 3z - 2 = 0 and x – 2y + 5 = 0
Here, a1 = 7, b1 = 5, c1 = 6 and a2 = 3, b2 = -1, c2 = -10
a1a2 + b1b2 + c1c2 = 7 * 3 + 5 * (-1) + 6 * (-10) = 21 – 5 – 60 = -44 ≠ 0
Therefore, the given planes are not perpendicular to each other.
a1a2 = , b1b2 =
= -5, c1c2 =
It can be seen that a1a2 ≠ b1b2 ≠ c1c2
Therefore, the given planes are not parallel to each other.
The angle between them is given by,
Q = cos-1
=> Q = cos-1
=> Q = cos-1
=> Q = cos-1
(b) The equations of the planes are 7x + 5y + 6z + 30 = 0 and 3x − y − 10z + 4 = 0
Here, a1 = 2, b1 = 1, c1 = 3 and a2 = 1, b2 = -2, c2 = 0
Now, a1a2 + b1b2 + c1c2 = 2 * 1 + 1 * (-2) + 3 * 0 = 2 – 2 + 0 = 0
Therefore, the given planes are perpendicular to each other.
(c) The equations of the planes are 2x - 2y + 4z + 5 = 0 and 3x – 3y + 6z - 1 = 0
Here, a1 = 2, b1 = -2, c1 = 4 and a2 = 3, b2 = -3, c2 = 6
Now, a1a2 + b1b2 + c1c2 = 2 * 3 + (-2) * (-3) + 4 * 6 = 6 + 6 + 24 = 36 ≠ 0
Therefore, the given planes are not perpendicular to each other.
a1a2 = , b1b2 =
, c1c2 =
It can be seen that a1a2 = b1b2 = c1c2
Therefore, the given planes are parallel to each other.
(d) The equations of the planes are 2x - y + 3z - 1 = 0 and 2x – y + 3z + 3 = 0
Here, a1 = 2, b1 = -1, c1 = 3 and a2 = 2, b2 = -1, c2 = 3
Here, a1a2 = = 1, b1b2 =
, c1c2 =
It can be seen that a1a2 = b1b2 = c1c2
Therefore, the given planes are parallel to each other.
(e) The equations of the planes are 4x + 8y + z - 8 = 0 and y + z - 4 = 0
Here, a1 = 4, b1 = 8, c1 = 1 and a2 = 0, b2 = 1, c2 = 1
Now, a1a2 + b1b2 + c1c2 = 4 * 0 + 8 * 1 + 1 * 1 = 0 + 8 + 1 = 9 ≠ 0
Therefore, the given planes are not perpendicular to each other.
Again, a1a2 = , b1b2 =
, c1c2 =
It can be seen that a1a2 ≠ b1b2 ≠ c1c2
Therefore, the given planes are not parallel to each other.
The angle between them is given by,
Q = cos-1
=> Q = cos-1
=> Q = cos-1
=> Q = cos-1
=> Q = cos-1
=> Q = 450