1/12+1/20+1/30+1/42+1/56
=1/（3×4）+1/（4×5）+1/（5×6）+1/（6×7）+1/（7×8）
=1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8
=1/3-1/8
=5/24