First, reproduce the wavefunction and condition for
eigenenergies as in Eq. (2), but for the case where
. ( Hint: If you are good with complex trigonometry, you
can save yourself some effort by noting what happens when you define
.)
Now, what does your condition on E become in the limit 
? Show that as the quantum number 
,
the energies you find are higher than the corresponding infinite
square well energies by 
.
Hint: You may find a graphical solution useful.