Problems

I have two problems (in P. F. Bernath, Spectra of toms and Molecules) to
solve, but I don't know how, so can anybody help me?

1. (7.22) Prove that the Fermi resonance cannot occur between two levels
with different values of l (vibrational angular momentum).

2. (7.25) (a) Using the vibrational constants calculate the symmetry and
frequency of all possible vibrational levels of HCN below 3500cm-1.
(b) Calculate all possible allwed transitions between these levels.

Thanks,
Nikola