Circles $x^2 + y^2 + 4x + d = 0, x^2 + y^2 + 4fy + d = 0$ touch each other, if
Circles $x^2 + y^2 + 4x + d = 0, x^2 + y^2 + 4fy + d = 0$ touch each other, if
- A
$f\,\, = \,\, \pm \,\,2\,\sqrt {4\,\, + \;\,d} $
- B
$f\,\, = \,\, \pm \,\,\frac{2}{{\sqrt {4\,\, - \,\,d} }}$
- C
$f\,\, = \,\, \pm \,\,\sqrt {\frac{d}{{\left( {4\,\, + \;\,d} \right)}}} $
- D
$f\,\, = \,\, \pm \,\,\sqrt {\frac{d}{{\left( {4\,\, - \,\,d} \right)}}} $
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