1 - 8/(x^2-4)/(1/x-1/2)*[1-(x^2+4)/4x]
=1-8/[(x+2)(x-2)]/(2-x)*2x*[(4x-x^2-4)/4x]
=1-8/[(x+2)(x-2)]/(2-x)*2x*[-(x-2)^2/4x]
1/（x-2）/(x-2)和(x-2)^2消去.1/4x和2x和8消去得4
负号提前为正
=1+4/(x+2)