import math


def width_min(b_2, a_thr):
    """Calculate minimum width and relative angle.

    :param b_2 (float): impeller width at section 2 [m]
    :param a_thr (float): area at throat section [m^2]
    :return b, theta (tuple): minimum volute width [m] and relative angle [rad]
    """
    b = round(1.5 * b_2, 3)
    theta = (math.pi * b)**2 / (2 * a_thr)

    return (b, theta)