1/12+1/20+1/30+1/40+.+1/110
=1/3*4+1/4*5+1/5*6+.+1/10*11
=（1/3-1/4）+（1/4-1/5）+（1/5-1/6）+.+（1/10-1/11）
=1/3-1/4+1/4-1/5+1/5-1/6+.+1/10-1/11
=1/3-1/11
=8/33