sn=f(2)-f(1)+f(3)-f(2)+.+f(n+1)-f(n)
=f(n+1)-f(1)
所以
lim(n→∞)sn
=lim(n→∞)[f(n+1)-f(1)]
=A-f(1)
所以级数收敛,且和=A-f(1)