1/11×13+1/13×15+1/15×17+1/17×19+1/38
=1/2×（1/11-1/13+1/13-1/15+1/15-1/17+1/17-1/19）+1/38
=1/2×（1/11-1/19）+1/38
=1/22-1/38+1/38
=1/22