(2001-1)+(2000-2)+(1999-3)+.+(1002-1000)
=(2001-1000)+(2000-999)+(1999-998)+.+(1002-1)
=1001+1001+1001+...+1001
=1001×1000
=1001000.
(2001-1)+(2000-2)+(1999-3)+.+(1002-1000)
=(2001-1000)+(2000-999)+(1999-998)+.+(1002-1)
=1001+1001+1001+...+1001
=1001×1000
=1001000.