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,
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Also, Perpendicular distance of the plane from origin = 1
We need to find the equation of such plane.
We know,
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Simplify the planes,
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⇒ x + 3y – 6 = 0 …(i)
Also, for
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⇒ 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
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Similarly, perpendicular distance of a plane (iii), which is equal to 1 (according to the question) is

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Squaring on both sides, we get
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⇒ (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
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⇒ 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.