def area_triangle_three_sides(a, b, c):
    """
    Formula: 0.25 * √( (a + b + c) * (-a + b + c) * (a - b + c)
    """
    return 0.25 * ((a + b + c) * (-a + b + c) * (a - b + c) * (a + b - c)
        ) ** 0.5
