1/13*15+1/15*17+1/17*19...+1/37*39
=(1/2)*(1/13-1/15+1/15-1/17+1/17-1/19+...+1/37-1/39)
=(1/2)*(1/13-1/39)
=(1/2)*2/39
=1/39这是固定思路1/n(n+2)=1/2×[1/n-1/(n+2)]然后前后就可以两两消掉了。。。