Q51 / 90 · JEE Main 2014 (06 Apr) · Mathematics

Let C  be the circle with center at 1,1 and radius =1. If T is the circle centered at 0,y, passing through the origin and touching the circle C externally, then the radius of T is equal to 

  • A
    1 2
  • Correct answer
    B
    1 4
  • C
    3 2
  • D
    3 2

Solution

Let r be the required radius
As origin lies on the circle, the distance of 0,0 from 0,y=r 

∴ y=r

Figure

As the two circles touch externally, distance between centres is sum of the radii

⇒0-12+y-12=r+1

⇒1+y2-2y+1=r2+2r+1

⇒1+r2-2r=r2+2r, ∵   y=r

∴1=4r

∴    r=14

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

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