def isen_nozzle_A_ratio(M_E, gamma_var):
    """
    Calculates ratio between exit and throat areas in an isentropic nozzle.

    Input variables:
        M_E         :   Mach number at exit
        gamma_var   :   Ratio of specific heats
    """

    A_ratio = (((gamma_var + 1) / 2)**-((gamma_var + 1) / \
        (2 * (gamma_var - 1)))) * ((1 + (((gamma_var - 1) / 2) * \
        (M_E**2)))**((gamma_var + 1) / (2 * (gamma_var - 1)))) * (1/M_E)

    return A_ratio