Write the molecular formula of the 2nd and the 3rd member of the homologous series whose first member is methane.

A homologous series is a family of organic compounds that share the same functional group and general chemical formula, with successive members differing by a $-CH_2-$ unit. For alkanes, the general formula is $C_nH_{2n+2}$. Methane ($CH_4$) is the first member ($n=1$). Adding a $-CH_2-$ unit gives the second member, ethane ($C_2H_6$), and adding another unit gives the third member, propane ($C_3H_8$).