分子在上,分母在下 应该是六分之五吧
3/2-5/6+7/12-9/20+11/30-13/42+15/56
=1+1/2-(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+1/2-1/2-1/3+1/3+1/4+.+1/7+1/8
=1+1/8
=9/8