3/1×2+5/2×3+7/3×4+9/4×5+11/5×6+13/6×7 =1+1/2+1/2+1/3+1/3+1/4+1/4+1/5+1/5+1/6+1/6+1/7=2+2/3+1/2+2/5+1/3+1/7=3+9/10+1/7=4又3/70 题目的+-符号有问题
1+1/2+1/2+1/3+1/3+1/4+……+1/6+1/7这个得通分吧
1249/20
