Resistance relation of parallel circuit Is it R1 + R2 divided by r1r2

Resistance relation of parallel circuit Is it R1 + R2 divided by r1r2

1 / R1 + 1 / r2 = 1 / R total
R total = r1r2 / (R1 + R2)

Resistance relation in series and parallel circuit

The total resistance in series is equal to the sum of all the resistances, that is, r = R1 + R2 +
The reciprocal of the total resistance in parallel is equal to the sum of the reciprocal of each branch resistance, i.e. 1 / r = 1 / R1 + 1 / r2 +

The relationship of resistance in parallel circuit
Resistance: the more parallel branches, the smaller the total resistance. The total resistance is less than that of any branch
1 / R total = 1 / R1 + 1 / r2 +. + 1 / RN
How is it derived?

R total = u total / I total = u / (I1 + I2 +...)
1 / R total = (I1 + I2 +...) )/U=I1/U+I2/U+… =1/R1+1/R2+…