4-2x=-2(x-2)
x^2-4=(x+2)(x-2)
∴最简公分母是2(x+2)(x-2)
x/(4-2x)=-x/2(x-2)=-x(x+2)/2(x+2)(x-2)
1/(x^2-4)=2/2(x+2)(x-2)