=2001.20012001+ 20012001*1 - 2001.20022002
=2001(20012001 - 20022002)+20012001
=2001*(-10001) +20012001
=2001*(-10001) +2001*10001
=2001(10001-10001)
=2001*0
=0
A=2002.(20010000+2001)-2001.(20020000+2002) =2002.(2001.10^4+2001)-2001.(2002.10^4+2001) =2002.2001.10^4+2002.2001-2001.2002.10^4-2001.2001=0