5/6-7/12+9/20-11/30+13/42-15/56+---+14/420-43/462
=1/2+1/3-1/3-1/4+1/4+1/5-1/5-1/6+1/6+1/7-1/7-1/8+…+1/20+1/21-1/21-1/22
=1/2-1/22
=5/11