1/2+1/6+1/12+1/20
=1/1*2+1/2*3+1/3*4+1/4*5
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)
=1-1/5
=4/5
累死了纯手打，不谢
