Q42 / 105 · NTA Abhyas (JEE Main) 2020 · Mathematics · Circle

If the length of direct common tangent and transverse common tangent of two circles with integral radii are 3 units and 1 unit respectively, then the reciprocal of the square of the distance between the centres of the circles is equal to

Correct answer: 0.1

Solution

Let two circles are of radii r1 & r2 and with centres C1 & C2 respectively, then
C1C22-r1-r22=9 &
C1C22-r1+r22=1
⇒4r1r2=8⇒r1r2=2
Assume, r1=2 & r2=1
Then, C1C22=1+32=10
1C1C22=0.1

Hence, the correct answer is 0.1.

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