1／2×5＋1／8×5＋1／8×11······﹢1／20×23
=1/3(1/2-1/5+1/5-1/8+1/8-1/11+1/11-1/4+1/14-1/17.+1/17-1/20+1/20-1/23)
=1/3(1/2-1/23)
=7/46