Write the function `resistor_parallel` that receive an undefined number of resistances parallel resistors and return the total resistance.
You can assume that there will be no 0 as parameter.  
Also there will be at least 2 arguments.
Formula:  
`total = 1 / (1/r1 + 1/r2 + .. + 1/rn)`
Examples:  
`resistor_parallel(20, 20)` will return `10.0`  
`resistor_parallel(20, 20, 40)` will return `8.0`
def resistor_parallel(*rs):
    return 1 / sum(1.0 / r for r in rs)
def resistor_parallel(*rs):
    return 1 / sum(1.0 / r for r in rs)