Update docs/2_debye_model_solutions.md

docs/2_debye_model_solutions.md

@@ -92,12 +92,10 @@ ax.legend();
 5. The density of states is the number of states per unit frequency. It has units of 1 over frequency
 
 ###  Exercise 2: Debye model in 2D.
-Here, we analyze the phonon energy and heat capacity of a two-dimensional Debye solid. 
-1. Formulate an integral expression for the energy stored in the vibrational modes of a two-dimensional Debye solid as a function of $T$.
+1. The energy stored in the vibrational modes of a two-dimensional Debye solid is:
     
     \begin{align}
     E & = \int_0^{\omega_D}(n_B(\omega(\mathbf{k}))+\frac{1}{2})\hbar\omega(\mathbf{k}) d\mathbf{k} \\    
-    & = \int_0^{\omega_D}(n_B(\omega(\mathbf{k}))+\frac{1}{2})\hbar\omega(\mathbf{k}) d\mathbf{k} +E_0\\
     & = \frac{L^2}{\pi v^2\hbar^2\beta^3}\int_{0}^{\beta\hbar\omega_D}\frac{x^2}{e^{x} - 1}dx + E_0
     \end{align}