2x³-6x-4=0
2x³+2x²-2x²-2x-4x-4=0 这一步用到了拆项法.
2x²(x+1)-2x(x+1)-4(x+1)=0
2(x+1)(x²-x-2)=0
2(x+1)(x+1)(x-2)=0
(x+1)²(x-2)=0
x=-1或x=2