1991×19901990-1990×19911991 等于多少?
问题描述：
1991×19901990-1990×19911991 等于多少?
答
1991×19901990-1990×19911991
=(1990+1)×19901990-1990×19911991
=1990×19901990+19901990-1990×19911991
=1990×（19901990-19911991）+19901990
=1990×(-10001)+19901990
=-19901990+19901990
=0