原式=1/(a-2)²-a/(a+2)(a-2)+1/2(a+2)
=[2(a+2)-2a(a-2)+(a-2)²]/2(a+2)(a-2)²
=(-a²+2a+8)/2(a+2)(a-2)²
=-(a-4)(a+2)/2(a+2)(a-2)²
=-(a-4)/2(a-2)²
当a=3时
原式=1/2