第1个回答 2020-10-08
y'''+6y''+(9+a^2)y'=1
The aux. equation
p^3 +6p^2 +(9+a^2)p=0
p[p^2+6p +(9+a^2) ]=0
p=0 or -3+ai or -3-ai
let
yg= C1 +e^(-3x) . [ C2.cos(ax) +C3.sin(ax) ]
let
yp= Ax
yp''' +6yp''+(9+a^2)yp'=1
A(9+a^2)=1
A=1/(9+a^2)
ie yp=[1/(9+a^2)]x
通解
y=yg+yp=C1 +e^(-3x) . [ C2.cos(ax) +C3.sin(ax) ] +[1/(9+a^2)]x