Two cells of e.m.f.s $E_1$ and $E_2$ ($E_1>E_2$) are connected as shown in figure. When the potentiometer is connected between A and B, the balancing length of the potentiometer wire is 3.60 m. On connecting the potentiometer between A and C, the balancing length is 0.90 m. The ratio $E_1/E_2$ is
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In the sum and difference method, $E_1/E_2 = (L_1 + L_2) / (L_1 - L_2)$ if connections are assisting and opposing.