def cosalpha(a, b, c):
    """
Calculate the cosign of an angle in a triangle a, b, c
    :param a: length of the side opposing the angle
    :param b: length of the side adjacent to the angle
    :param c: length of the side adjacent to the angle
    :return: cosign
    """
    return (b ** 2 + c ** 2 - a ** 2) / (2 * b * c)