import cmath


def descartesEq(k1, k2, k3):
    """ Solve the Descartes Theorem. """
    # "When trying to find the radius of a fourth circle tangent to three
    # given kissing circles, the equation is best rewritten as:"
    # <https://en.wikipedia.org/wiki/Descartes%27_theorem>
    return k1 + k2 + k3 + 2*(cmath.sqrt(k1 * k2 + k2 * k3 + k1 * k3))