把每一个分式分成二个就行了如1/（a-1)a=1/(a-1)-1/a1/a(a+1)=1/a-1/(a+1)........1/(a+n-1)(a+n)=1/(a+n-1)-1/(a+n)然后两边分别相加即可。左边=1/(a-1)-1/(a+n)
