A) ? 4
B) 4
C) 0
D) 1
Correct Answer: A
Solution :
Circle \[{{x}^{2}}+{{y}^{2}}+2gx+2fy+c=0\] is concentric with \[{{x}^{2}}+{{y}^{2}}-2x+4y+20=0\]. Therefore centre is \[(1,\ -2)\] and radius \[=\sqrt{{{g}^{2}}+{{f}^{2}}-c}=\sqrt{{{(1-4)}^{2}}+{{(-2+2)}^{2}}}\]\[\Rightarrow c=-4\].
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