Solution
= Solution
The homogeneous <characteristic equation> is $r^2+9=0$, with roots $r=\pm3i$. Its real <complementary function> is
$$
\boxed{y_c=C_1\cos3x+C_2\sin3x}.
$$
= Solution
The homogeneous <characteristic equation> is $r^2+9=0$, with roots $r=\pm3i$. Its real <complementary function> is
$$
\boxed{y_c=C_1\cos3x+C_2\sin3x}.
$$