The current (I) flowing through R1 and R2 is the same
and so the potential differences across them are V1 = IR1 and
V2 = IR2
But using Kirchoff's second rule the total
potential difference across them is V = V1 +
V2 = IR = IR1 + IR2 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 =
I1 + I2 = V/R =
V/R1 + V/R2
where R is the effective resistance of the two resistors in parallel.