Two sources of equal emf are connected to an external resistance R. The internal resistances 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=R1R2/(R1+R2)

(2) R=R1R2/(R2R1)

(3) R=R2×(R1+R2)/(R2R1)

(4) R=R2R1

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