`a``)` `Ta` `có``:` $10^{21}$ `=` `1000000000000000000000` `(` `Có` `21` `chữ` `số` `0` `)`
`⇒` `1988550` `.` $10^{21}$ `=` `1988550000000000000000000000``.`
`b``)` `Theo` `câu` `a` `ta` `có``:` $10^{21}$ `=` `1000000000000000000000` `(` `Có` `21` `chữ` `số` `0` `)`
`⇒` `6` `.` $10^{21}$ `=` `6000000000000000000000``.`