其特征方程为x^2+9=0,x=-3i或x=3i则其通解为y=C1cos3x+ C2sin3x再求一个特y''+9y=-6cos3x-6sin3x的特解y=y1+y2y1为y''+9y=-6cos3x的特解y2为y''+9y=-6sin3x的特解y1=1/(D^2+9)*(-6cos3x）=-x*sin3xy2=1/(D^2+9)*(-6s...