Find the equation of the plane through the intersection of the planes and whose perpendicular distance from origin is unity.
We are given with two planes,
Also, Perpendicular distance of the plane from origin = 1
We need to find the equation of such plane.
We know,
Simplify the planes,
⇒ x + 3y – 6 = 0 …(i)
Also, for
⇒ 3x – y – 4z = 0 …(ii)
We know that,
The equation of a plane through the line of intersection of the planes a1x + b1y + c1z + d1 = 0 and a2x + b2y + c2z + d2 = 0 is given by
(a1x + b1y + c1z + d1) + λ(a2x + b2y + c2z + d2) = 0
Similarly, equation of a plane through the line of intersection of the planes x + 3y – 6 = 0 and 3x – y – 4z = 0 is
(x + 3y – 6) + λ(3x – y – 4z) = 0
⇒ x + 3y – 6 + 3λx – λy – 4λz = 0
⇒ x + 3λx + 3y – λy – 6 – 4λz = 0
⇒ (1 + 3λ)x + (3 – λ)y – 4λz – 6 = 0 …(iii)
Also, we know that
Perpendicular distance of a plane, ax + by + cz + d = 0 from the origin is P, such that
Similarly, perpendicular distance of a plane (iii), which is equal to 1 (according to the question) is
Squaring on both sides, we get
⇒ (1 + 3λ)2 + (3 – λ)2 + (-4λ)2 = 36
⇒ (1)2 + (3λ)2 + 2(1)(3λ) + (3)2 + (λ)2 – 2(3)(λ) + 16λ2 = 36
[∵, (a + b)2 = a2 + b2 + 2ab and (a – b)2 = a2 + b2 – 2ab]
⇒ 1 + 9λ2 + 6λ + 9 + λ2 – 6λ + 16λ2 = 36
⇒ 9λ2 + 16λ2 + λ2 + 6λ – 6λ = 36 – 1 – 9
⇒ 26λ2 + 0 = 26
⇒ 26λ2 = 26
⇒ λ2 = 1
⇒ λ = ± 1
First, substitute λ = 1 in equation (iii) to find the equation of the plane.
(1 + 3λ)x + (3 – λ)y – 4λz – 6 = 0
⇒ (1 + 3(1))x – (3 – 1)y – 4(1)z – 6 = 0
⇒ 4x – 2y – 4z – 6 = 0
Now, substitute λ = -1 in equation (iii) to find the equation of the plane.
(1 + 3λ)x + (3 – λ)y – 4λz – 6 = 0
⇒ (1 + 3(-1))x + (3 – (-1))y – 4(-1)z – 6 = 0
⇒ (1 – 3)x + (3 + 1)y + 4z – 6 = 0
⇒ -2x + 4y + 4z – 6 = 0
Thus, equation of the required plane is 4x – 2y – 4z – 6 = 0 and -2x + 4y + 4z – 6 = 0.