Q59 / 90 · KVPY 2019 (SB/SX) · Mathematics

Let a=i^+j^+k^,b=2i^+2j^+k^ and c=5i^+j^-k^ be three vectors. The area of the region formed by the set of points whose position vectors r→ satisfy the equations r·a=5  and |r-b|+|r-c|=4 is closest to the integer.

  • Correct answer
    A
    4
  • B
    9
  • C
    14
  • D
    19

Solution

Since, |r-b|+|r-c|=4 is equation of ellipsoid in plane r,a=5 or x+y+z=5.

∴ Distance between foci =|b-c|

=|3i^-j^-2k^|=14=2ae

and 2a=4

∴ e=144⇒e2=1-b2a2

⇒1416=1-b24⇒b24=216

b2=12

∴ Area of ellipsoid 

=πab=π212=2π

And from the given options the closest integer to 2π is 4.

Hence, the correct answer is option: 'A'.

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