#### Resistor Equivalence

G
```# https://byjus.com/equivalent-resistance-formula/

from __future__ import annotations

def resistor_parallel(resistors: list[float]) -> float:
"""
Req = 1/ (1/R1 + 1/R2 + ... + 1/Rn)

>>> resistor_parallel([3.21389, 2, 3])
0.8737571620498019
>>> resistor_parallel([3.21389, 2, -3])
Traceback (most recent call last):
...
ValueError: Resistor at index 2 has a negative or zero value!
>>> resistor_parallel([3.21389, 2, 0.000])
Traceback (most recent call last):
...
ValueError: Resistor at index 2 has a negative or zero value!
"""

first_sum = 0.00
index = 0
for resistor in resistors:
if resistor <= 0:
raise ValueError(f"Resistor at index {index} has a negative or zero value!")
first_sum += 1 / float(resistor)
index += 1
return 1 / first_sum

def resistor_series(resistors: list[float]) -> float:
"""
Req = R1 + R2 + ... + Rn

Calculate the equivalent resistance for any number of resistors in parallel.

>>> resistor_series([3.21389, 2, 3])
8.21389
>>> resistor_series([3.21389, 2, -3])
Traceback (most recent call last):
...
ValueError: Resistor at index 2 has a negative value!
"""
sum_r = 0.00
index = 0
for resistor in resistors:
sum_r += resistor
if resistor < 0:
raise ValueError(f"Resistor at index {index} has a negative value!")
index += 1
return sum_r

if __name__ == "__main__":
import doctest

doctest.testmod()
```  