Update 8_many_atoms_solutions.md - typo fix

src/8_many_atoms_solutions.md

@@ -125,7 +125,7 @@ $$
 and for the effective mass we obtain
 
 $$
-m^*(k) = \mp \frac{\hbar^2}{a^2 t_1 t_2}\left[ \frac{t_1 t_2 \sin^2(ka)}{(t_1^2 + t_2^2 +2 t_1 t_2\cos(ka))^{3/2}} + \frac{\cos(ka)}{\sqrt{t_1^2 + t_2^2 +2 t_1 t_2\cos(ka)}}  \right]. 
+m^*(k) = \mp \frac{\hbar^2}{a^2 t_1 t_2}\left[ \frac{t_1 t_2 \sin^2(ka)}{(t_1^2 + t_2^2 +2 t_1 t_2\cos(ka))^{3/2}} + \frac{\cos(ka)}{\sqrt{t_1^2 + t_2^2 +2 t_1 t_2\cos(ka)}}  \right]^{-1}. 
 $$
 
 6.