y'' - 3y' - 4y = 5r^2 - 3r - 4 = 0(r - 4)(r + 1) = 0r = - 1 或 r = 4y' = Ae^(- x) + Be^(4x) + P设P = CP' = P'' = 0,代入y'' - 3y - 4y = 50 + 0 - 4C = 5 ==> C = - 5/4y = Ae^(- x) + Be^(4x) - 5/4