x²-2x/x²-1÷[x-1-(2x-1)/(x+1)]
=x²-2x/x²-1÷(x²-1-2x+1)/(x+1)
=x²-2x/x²-1×(x+1)/x²-2x
=1/x-1
∵x=2分之1
∴1/x-1=-2