原式=(X^2+1)/{x+1)(x-1)} -(X-2)/(X-1) *x /(X-2)
=(X^2+1)/{x+1)(x-1)} -x/(X-1)
={(X^2+1)-x(x+1)}/{(x+1)(X-1)}
=(X^2+1-x^2-x)/{(x+1)(X-1)}
=(1-x)/{(x+1)(X-1)}
=-(x-1)/{(x+1)(X-1)}
=-1/(x+1)