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CBSE Class 12 Math 2009 Solved Paper

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Question : 27 of 29
Marks: +1, -0
Find the equation of the plane passing through the point (-1, 3, 2) and perpendicular to each of the planes x + 2y + 3z = 5 and 3x + 3y + z = 0.
Solution:  
Let the equation of the plane be,
A (x−x1)(x-x_1) + B (y−y1)(y-y_1) + C (z−z1)(z-z_1) = 0
Plane passes through the point (-1, 3, 2)
∴ A (x + 1) + B (y - 3) + C (z - 2) = 0 ... (i)
Now applying the condition of perpendicularity to the plane (i) with planes
x + 2y + 3z = 5 and 3x + 3y + z = 0, we have
A + 2B + 3C = 0
3A + 3B + C = 0
Solving we get
A + 2B + 3C = 0
9A + 9B + 3C = 0
By cross multiplication, we have,
A2×3−9×3\frac{A}{2\times 3-9\times 3} = B9×3−1×3\frac{B}{9\times 3-1\times 3} = C1×9−2×9\frac{C}{1\times 9-2\times 9}
⇒ A6−27\frac{A}{6-27} = B27−3\frac{B}{27-3} = C9−18\frac{C}{9-18}
⇒ A−21\frac{A}{-21} = B24\frac{B}{24} = C−9\frac{C}{-9}
⇒ A7\frac{A}{7} = B−8\frac{B}{-8} = C3\frac{C}{3}
⇒ A = 7λ ; B = - 8λ ; C = 3λ
By substituting A and C in equation (i), we get,
Substituting the values of A, B and C in equation (i), we have,
7λ (x + 1) - 8λ (y - 3) + 3λ (z - 2) = 0
⇒ 7x + 7 - 8y + 24 + 3z - 6 = 0
⇒ 7x - 8y + 3z + 25 = 0
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