Update src/7_tight_binding_model_sol.md

src/7_tight_binding_model_sol.md

@@ -69,8 +69,9 @@ For the energy we have: $$U = \int \hbar \omega g(\omega) (n(\omega,T) + \frac{1
 
 ### Subquestion 2
 
+For the heat capacity we have: $$C = \frac{\partial U}{\partial T} = \int g(\omega) \hbar\omega \frac{\partial n(\omega,T)}{\partial T}d\omega$$
 
-For the heat capacity we have: $$C = \frac{\partial U}{\partial T} = \int \hbar\omega g(\omega) \frac{\partial n(\omega,T)}{\partial T}d\omega$$
-
+## Exercise 3: Next-nearest neighbors chain
 
+### Subquestion 1