1-x²=(1-x)(1+x)x²-2x+1=(x-1)²x²+x-2=(x-1)(x+2)所以：最简公分母为(x-1)²(x+1)(x+2)
1-x²=(1+x)(1-x)x²-2x+1=(1-x)(1-x)x²+x-2=(x-1)(x+2)最简公分母(x-1)²(x+2)(x+1)
(1-x²) (x²-2x+1)( x²+x-2 )
