常数变易法求二阶通解

特征方程 r² + 5 r + 6 = 0

特征解 r = -5/2 ± i /2

非齐次方程特解设为 e^(-5t/2) [ u(t) cos(t/2) + v(t) sin(t/2) ]

原方程通解 y = e^(-5t/2) [ C1 cos(t/2) + C2 sin(t/2) ] + e^(-5t/2) [ u(t) cos(t/2) + v(t) sin(t/2) ]