1.(a+2)/(a+1)-(a+3)/(a+2)=1/(a+1)(a+2),(a-4)/(a-3)-(a-5)/(a-4)=1/(a-3)(a-4).所以原式=1/(a+1)(a+2)-1/(a-3)(a-4)=-10（a+1）/(a+1)(a+2)(a-3)(a-4）=-10/(a+2)(a-3)(a-4）