(1/8+1/24+1/48+1/80+1/120+168/1)/56
=[1/(2×4)+1/(4×6)+1/(6×8)+1/(8×10)+1/(10×12)+1/(12×14)] ×1/56
=1/2×[(1/2－1/4)+（1/4－1/6)+（1/6－1/8)+（1/8－1/10)+（1/10－1/12)+（1/12－1/14)]×1/56
=1/2×[1/2－1/4+1/4－1/6+1/6－1/8+1/8－1/10+1/10－1/12+1/12－1/14]×1/56
=1/2×[1/2－1/14]×1/56
=1/2×6/14×1/56
=3/784