│1/则圆21-1/20│+│1/21-1/22│+|1/23-1/22|+...+|1/30-1/29|
=1/20 -1/厅盯基21 +1/21-1/22 +1/22-1/23+......+1/29-1/30
=1/20 -1/扮谨30
=1/60
│1/隐李21-1/20│+│1/慧神21-1/22│+|1/23-1/22|+...+|1/30-1/29|
=1/20-1/21+1/21-1/22+...+1/29-1/29+1/29-1/30
=1/20+(-1/21+1/灶碧迟21)+(-1/22+1/22)+....+(-1/29+1/29)-1/30
=1/20-1/30
=1/60