1/x^2-1
=1/(x-1)(x+1),
=x(x-1)/x(x-1)^2(x+1),
1/x^2-x,
=1/x(x-1)
=(x-1)(x+1)/x(x-1)^2(x+1)
1/x^2-2x+1
=1/(x-1)^2
=x(x+1)/x(x-1)^2(x+1)