An infinite ladder network is arranged with resistances $R$ and $2 R$ as shown. The effective resistance between terminals $A$ and $B$ is
A$\infty$
B$R$
C$2 R$
D$3 R$
Diffcult
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C$2 R$
c (c) Let equivalent resistance between $A$ and $B$ is $R'$, so given circuit can be reduced as follows
$R' = R + \frac{{2R \times R'}}{{(2R + R')}} \Rightarrow {R'^2} - RR' - 2{R^2} = 0$
On solving the equation we get $R' = 2R$.
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