Condition for two circles to touch: find the parameter

1. A.
   $\frac{27}{64}$
2. B.
   $\frac{125}{27}$
3. C.
   $\frac{27}{125}$
4. D.
   $\frac{64}{27}$

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1. A.
   $\frac{1}{a^2}-\frac{1}{b^2}=\frac{1}{c}$
2. B.
   $\frac{1}{a^2}+\frac{1}{b^2}=\frac{1}{c}$
3. C.
   $\frac{1}{a^2}+\frac{1}{b^2}=\frac{1}{c^2}$
4. D.
   $\frac{1}{a^2}-\frac{1}{b^2}=\frac{1}{c^2}$

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1. A.
   $\dfrac{2}{3}$
2. B.
   $\dfrac{1}{56}$
3. C.
   $\dfrac{-2}{3}$
4. D.
   $\dfrac{3}{2}$

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