(2x-4)/x^2-4x+4
=2(x-2)/(x-2)^2
=2/(x-2)
1-4/2-x
=1+4/x-2
=(x-2+4)/x-2
=(x+2)/x-2
原式=2/(x-2)÷(x+2)/x-2=2/(x+2)