The ratio of specific heats for an ideal gas is given by (Symbols have the usual meanings)
1
\(\frac{{1}}{{1\;-\;\frac{{C_p}}{{R}}}}\)
2
\(\frac{{1}}{{1\;-\;\frac{{R}}{{C_p}}}}\)
3
\(\frac{{1}}{{1\;+\;\frac{{C_p}}{{R}}}}\)
4
\(\frac{{1}}{{1\;+\;\frac{{R}}{{C_p}}}}\)