1/2加1/6加1/12加1/20加1/30
=1-1/2+1/2-1/3+1/4-1/5+1/5-1/6
=1-1/6
=5/6讲解一下加加加1/20加1/301/2=1-1/21/6=1/2-1/31/12=1/3-1/41/20=1/4-1/51/30=1/5-1/6