对应齐次方程y′′ + 9 y = 0
特征方程λ^2 + 9 = 0,特征根λ =3i or -3i
齐次方程的通解为y = C1 cos3 x + C2 sin3 x
设y p = Acos2x +Bsin2x
Yp的一阶= -2Asin2x +2Bcos2x
Yp的二阶=-4Acos2x -4Bsin2x
代入原式有5Acos2x+5Bsin2x=5cos2x
A=1B=0
因此解为y = C1 cos3 x + C2 sin3 x+cos2x
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