1990+19901990+199019901990+…/1991+19911991+199119911991+…
=【1990×(1+10001+100010001+……)】/【1991×(1+10001+100010001)】
=1990/1991
1990+19901990+199019901990+…/1991+19911991+199119911991+…
=【1990×(1+10001+100010001+……)】/【1991×(1+10001+100010001)】
=1990/1991