6.6*99+6.6*10=6.6（100-1）+66=660-6.6+66=726-6.6=719.4
6.6x99+6.6xI0=6.6（99+10）=6.6x（100+9）=660+59.4=719.4
＝6.6×（99+1）+6.6×9＝660+59.4＝719.4
