4/(a^2-4)+2/(a+2)-1/(a-2) =4/(a+2)(a-2)+2(a-2)/(a+2)(a-2)-(a+2)/(a+2)(a-2) =[4+2a-4-a-2]/(a+2)(a-2) =(a-2)/(a+2)(a-2) =1/(a+2) 当a=根号2-2时,原式=1/(根号2)=（根号2）/2
