What’s the formula for 2/3/4 parallel resistors?

This post is also available in: deDeutsch (German)

Formula for 2 parallel resistors R1 and R2:

R_{total} = \frac{1}{\frac{1}{R_1} + \frac{1}{R_2}}

Python code:

rtotal = 1. / (1./r1 + 1./r2)

Formula for 3 parallel resistors R1 and R2:

R_{total} = \frac{1}{\frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3}}

Python code:

rtotal = 1. / (1./r1 + 1./r2 + 1./r3)

Formula for 4 parallel resistors R1 and R2:

R_{total} = \frac{1}{\frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + \frac{1}{R_4}}

Python code:

rtotal = 1. / (1./r1 + 1./r2 + 1./r3 + 1./r4)