Solution
= Solution
Release from rest gives $B=D=0$. At $t=0$, $A+2C=d$ and $A-C=0$, so $A=C=d/3$. Consequently
$$
\boxed{y_1(t)=\frac d3\bigl(\cos\omega_-t+2\cos2\omega_-t\bigr),\qquad
y_2(t)=\frac d3\bigl(\cos\omega_-t-\cos2\omega_-t\bigr).}
$$
The <frequency ratio> is exactly two, so both motions repeat after $2\pi/\omega_-$. These are commensurate <normal modes>, rather than a pair of nearly equal frequencies producing slow <beats>.