x/（2-x）-(4-x^2)/（x^2+4x+4 ）
=x/(2-x)+(x+2)(x-2)/[(x+2)²]
=x/(x-2)+(x-2)/(x+2)
=[x(x+2)+x(x-2)]/[(x-2)(x+2)]
=2x²/(x²-4)