1/35+1/63+1/99+1/143+1/195
=1/5×7+1/7×9+1／9×11＋1／11×13＋1／13×15
=1/2[(1/5-1/7)+(1/7-1/9)+(1/9-1/11)+(1/11-1/13)+(1/13-1/15)]
=1/2[1/5-1/7+1/7-1/9+1/9-1/11+1/11-1/13+1/13-1/15]
=1/2[1/5-1/15]
=1/2×2/15
=1/15