NCERT Solutions
Class 12 Maths
Three Dimensional Geometry

Ex.11.3 Q.5
Find the vector and Cartesian equation of the planes
(a) that passes through the point (1, 0, −2) and the normal to the plane is i + j - k.
(b) that passes through the point (1, 4, 6) and the normal vector to the plane is i – 2j + k.
(a) The position vector of point (1, 0, −2) is a = i – 2k
The normal vector N perpendicular to the plane is N = i + j - k
The vector equation of the plane is given by,
(r - a).N = 0
=> [r – (i – 2k)].(i + j - k) = 0 …………1
r is the position vector of any point P(x, y, z) in the plane.
So, r = xi + yj + zk
Therefore, equation 1 becomes
[(xi + yj + zk) – (i – 2k)].(i + j - k) = 0
=> [(x – 1)i + yj + (z + 2)k] .(i + j - k) = 0
=> (x – 1) + y - (z + 2) = 0
=> x + y – z – 3 = 0
=> x + y – z = 3
This is the Cartesian equation of the required plane.
(b) The position vector of the point (1, 4, 6) is a = i + 4j + 6k
The normal vector perpendicular to the plane is N = i – 2j + k
The vector equation of the plane is given by,
(r - a).N = 0
=> [r – (i + 4j + 6k)].(i - 2j + k) = 0 …………1
r is the position vector of any point P(x, y, z) in the plane.
So, r = xi + yj + zk
Therefore, equation 1 becomes
[(xi + yj + zk) – (i + 4j + 6k)].(i - 2j + k) = 0
=> [(x – 1)i + (y - 4)j + (z - 6)k] .(i - 2j + k) = 0
=> (x – 1) – 2(y - 4) + (z - 6) = 0
=> x - 2y + z + 1 = 0
This is the Cartesian equation of the required plane.