裂项的方法操作:分母可以拆成2x3,3x4,4x5,5x6,6x7,7x8再裂项得1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8计算得1/2-1/8+=3/8
1/6+1/12+1/20+1/30+1/42+1/56=1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8=1/2-1/8=3/8
