3D Geometry 3 Question 6

6. If the plane 2xy+2z+3=0 has the distances 13 and 23 units from the planes 4x2y+4z+λ=0 and 2xy+2z+μ=0, respectively, then the maximum value of λ+μ is equal to

(2019 Main, 10 April II)

(a) 13

(b) 15

(c) 5

(d) 9

Show Answer

Answer:

(a)

Solution:

  1. Equation of given planes are

2xy+2z+3=0

4x2y+4z+λ=0

and

2xy+2z+μ=0

Distance between two parallel planes

ax+by+cz+d1=0 and ax+by+cz+d2=0 is 

distance =|d1d2|a2+b2+c2

Distance between planes (i) and (ii) is

|λ2(3)|16+4+16=13

[given]

|λ6|=2λ6=±2λ=8 or 4 and distance between planes (i) and (iii) is

|μ3|4+1+4=23|μ3|=2μ3=±2μ=5 or 1

So, maximum value of (λ+μ) at λ=8 and μ=5 and it is equal to 13.



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