y''-9(y+1)=0(y+1)''-9(y+1)=0(y+1)=uu''-9u=0特征方程r^2-9=0r1=3,r2=-3u=C1e^3x+C2e^(-3x)通解y+1=C1e^(3x)+C2e^(-3x)
