$(a)$ What is the total resistance of $n$ resistors each of resistance ' $R^{\prime}$ connected in

$(i)$ series ?

$(ii)$ parallel ?

$(b)$ Calculate the resultant resistance of $3$ resistors $3\, \Omega, 4\, \Omega$ and $12\, \Omega$ connected in parallel.

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$(a)$ In series combination, we have

$R_{S}=R_{1}+R_{2}+\ldots$ to $n=n R$

In parallel combination, we have

$\frac{1}{ R _{ P }}=\frac{1}{ R _{1}}+\frac{1}{ R _{2}}+\ldots n$

Or $R _{ P }= R / n$

$(b)$ In parallel, we have

$\frac{1}{ R _{ P }}=\frac{1}{ R _{1}}+\frac{1}{ R _{2}}+\frac{1}{ R _{3}}=\frac{1}{3}+\frac{1}{4}+\frac{1}{12}$

or $\quad R_{P}=12 / 8=1.5 \Omega$

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