裂项法：
1/15+1/35+1/63+1/99+1/143+1/195
=1/2×（1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11+1/11-1/13+1/13-1/15）
=1/2×（1/3-1/15）
=1/2×4/15
=2/15