2a/(a^2-4)-1/a+2=2a/(a+2)(a-2)-1/a+2,其中2a/(a^2-4)=1/(a+2)+1/(a-2)=1/(a+2)+1/(a-2)-1/a+2=1/(a-2)
=2a/(a²-4)-(a-2)/(a²-4)=(a+2)/(a²-4)=1/(a-2)