︱ 1/1001-1/1000︱+︱ 1/1002-1/1001︱-︱ 1/1002-1/1000︱
=1/1000-1/1001+1/1001-1/1002-1/1000+1/1002
=(1/1000-1/1000)+(1/1001-1/1001)+(1/1002-1/1002)
=0