changes x to u for consistency

src/6_bonds_and_spectra_solutions.md

@@ -29,9 +29,9 @@ $$
 1.  In 1D, there are two normal modes and in 3D there are 4 normal modes
 2.  $$
 \begin{cases}
-    m \ddot{x}_1 & = -\kappa(x_1-x_2)\\
-    M \ddot{x}_2 & = -\kappa(2x_2-x_1-x_3)\\ 
-    m \ddot{x}_3 & = -\kappa(x_3-x_2)
+    m \ddot{u}_1 & = -\kappa(u_1-u_2)\\
+    M \ddot{u}_2 & = -\kappa(2u_2-u_1-u_3)\\ 
+    m \ddot{u}_3 & = -\kappa(u_3-u_2)
 \end{cases}
 $$
 Where $m$ is the mass of the oxygen atoms and $M$ the mass of the carbon atom.
@@ -39,7 +39,7 @@ Where $m$ is the mass of the oxygen atoms and $M$ the mass of the carbon atom.
 \omega = \sqrt{\frac{\kappa}{m}}
 $$
 4.  $$
-\frac{x_1}{x_2} = -\frac{M}{2m}
+\frac{u_1}{u_2} = -\frac{M}{2m}
 $$
 5.  $$
 \omega = \sqrt{\frac{\kappa(2m+M)}{mM}}