from typing import Callable

def bisect( f: Callable[ [ float ], float ],
            x_1: float, x_2: float, prec: float ) -> float:

    mid = ( x_1 + x_2 ) / 2

    if abs( x_1 - x_2 ) < 2 * prec:
        return mid

    if f( mid ) * f( x_1 ) <= 0:
        return bisect( f, x_1, mid, prec )
    else:
        return bisect( f, mid, x_2, prec )

import run_tests