x三次方-x+x+1分之1-x四次方
=x(x²-1)-(x²+1)(x²-1)/(x+1)
=x(x²-1)-(x²+1)(x+1)(x-1)/(x+1)
=x(x+1)(x-1)-(x²+1)(x-1)
=(x²+x-x²-1)(x-1)
=(x-1)²