fix(ch6): correct forced vibration coefficient collection

chapters/ch06_mechanical_applications.tex

@@ -441,16 +441,16 @@ Differentiating:
 
 Substitute into \cref{eq:forced_vibration}:
 \[
-  \bigl[-m\omega^2 A - c\omega\omega B + kA\bigr]\cos(\omega t)
-  + \bigl[-m\omega^2 B + c\omega A + kB\bigr]\sin(\omega t)
+  \bigl[-m\omega^2 A + c\omega B + kA\bigr]\cos(\omega t)
+  + \bigl[-m\omega^2 B - c\omega A + kB\bigr]\sin(\omega t)
   = F_0\cos(\omega t).
 \]
 
 Equating coefficients:
 \[
   \begin{cases}
-    (k - m\omega^2)A - c\omega B = F_0, \\
-    c\omega A + (k - m\omega^2)B = 0.
+    (k - m\omega^2)A + c\omega B = F_0, \\
+    -c\omega A + (k - m\omega^2)B = 0.
   \end{cases}
 \]