Add solutions to excercise 3 in lecture 1.

src/solutions/1_einstein_model.md

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 where we used $\sum_{n = 0}^{\infty}nr^n = \frac{r}{(1 - r)^2}$.
 
 ### Exercise 3: Total heat capacity of a diatomic material.
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+1.
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+Use the formula $\omega = \sqrt{\frac{k}{m}}$.
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+2.
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+$E = \frac{N_{^6Li}}{N}\hbar\omega_{^6Li}(2 + 1/2)+\frac{N_{^7Li}}{N}\hbar\omega_{^7Li}(4 + 1/2)$.
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+3.
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+$E = \hbar\omega_{^6Li}\left(n_B(\beta\hbar\omega_{^6Li}) + \frac{1}{2}\right) + \hbar\omega_{^7Li}\left(n_B(\beta\hbar\omega_{^7Li}) + \frac{1}{2}\right)$.
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+4.
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+$C = C_{^6Li} + C_{^7Li}$ where the heat capacities are calculated with the formula from Excercise 2.4.