用matlab给的解析解~
x1 = 0
x2 = -2/3*(27+3*105^(1/2))^(1/3)+4/(27+3*105^(1/2))^(1/3)
x3 = 1/3*(27+3*105^(1/2))^(1/3)-2/(27+3*105^(1/2))^(1/3) + i*3^(1/2)*(-1/3*(27+3*105^(1/2))^(1/3)-2/(27+3*105^(1/2))^(1/3))
x4 = 1/3*(27+3*105^(1/2))^(1/3)-2/(27+3*105^(1/2))^(1/3) - i*3^(1/2)*(-1/3*(27+3*105^(1/2))^(1/3)-2/(27+3*105^(1/2))^(1/3))
转化成数值解为：
x1=0
x2=-1.5418
x3=0.7709 - 3.1278i
x4=0.7709 + 3.1278i