The current (I) flowing through R_{1} and R_{2} is the same
and so the potential differences across them are V_{1} = IR_{1} and
V_{2} = IR_{2}

But using Kirchoff's second rule the total
potential difference across them is V = V_{1} +
V_{2} = IR = IR_{1} + IR_{2} where R is the effective series resistance of the two resistors.

The potential difference (V)
across each of the two resistors is the same, and the current (I) flowing into junction A is
equal to the sum of the currents in the two branches (Kirchoff's first rule) therefore:

I =
I_{1} + I_{2} = V/R =
V/R_{1} + V/R_{2}

where R is the effective resistance of the two resistors in parallel.