特解为：(x^3*exp(4*x))/9 - (exp(4*x)*(9*x^2 - 6*x + 2))/81通解为：(x^3*exp(4*x))/9 - (exp(4*x)*(9*x^2 - 6*x + 2))/81 + C1*exp(x) + C2*exp(4*x)C1，C2均为常数.
