Two sources of equal emf are connected to an external resistance R. The internal resistance of the two sources are R1 and R2 (R2 > R1). If the potential difference across the source having internal resistance R2 is zero, then
(1) R = R2 × (R1 + R2)/R2 – R1)
(2) R = R2 – R1
(3) R = R1R2 / (R1 + R2)
(4) R = R1R2 / (R2 – R1)
