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Q.
Two resistors $R _{1}=(4 \pm 0.8) \,\Omega$ and $R _{2}=(4 \pm 0.4) \,\Omega$ are connected in parallel. The equivalent resistance of their parallel combination will be :
$\frac{1}{R_{\text{eq}}}=\frac{1}{R_{1}}+\frac{1}{R_{2}}$
$\frac{1}{R_{\text{eq}}}=\frac{1}{4}+\frac{1}{4} $
$\Rightarrow R_{\text{eq}}=2 \Omega$
Also $\frac{\Delta R _{\text{eq}}}{ R _{ \text{eq }}^{2}}=\frac{\Delta R _{1}}{ R _{1}^{2}}+\frac{\Delta R _{2}}{ R _{2}^{2}}$
$\frac{\Delta R _{\text{eq}}}{4}=\frac{.8}{16}+\frac{.4}{16}=\frac{1.2}{16}$
$\underline{\underline{\Delta}} R _{\text{eq}}=0.3 \Omega$
$R _{\text{eq}}=(2 \pm 0.3) \Omega$