def Gamma1_gasrad(beta):
Gamma3minus1 = (old_div(<NUM_LIT>,<NUM_LIT>))*(<NUM_LIT>-<NUM_LIT>*beta)/(<NUM_LIT>-<NUM_LIT>*beta) <EOL>Gamma1 = beta + (<NUM_LIT>-<NUM_LIT>*beta) * Gamma3minus1<EOL>return Gamma1<EOL>
Gamma1 for a mix of ideal gas and radiation

Hansen & Kawaler, page 177, Eqn. 3.110

Parameters
----------
beta : float
    Gas pressure fraction Pgas/(Pgas+Prad)