import math


def _is_external(a, b):
    """ Check if blob b is outside blob a

    :param a: first blob. This is larger than b.
    :param b: second blob. This is smaller than a.
    :returns: if b is outside the radius of a
    """
    ay, ax, ar = a
    by, bx, br = b

    d = math.sqrt((ax - bx)**2 + (ay - by)**2)
    return d >= ar + br