解方程：1除以x-1-1=3除以(x+1)(x+2)

1/(x-1)-1=3/[(x+1)(x+2) ]
(2-x)(x+1)(x+2)=3(x-1) (最简公分母是(x-1)(x+1)(x+2))
(4-x²)(x+1)=3x-3
x³+x²-x=0
x(x²+x-1)=0
x1=0
x2=(√5-1)/2
x3=(-√5-1)/2
经代入最简公分母是(x-1)(x+1)(x+2)均不为02-x怎么得出来的？