7.8*99+0.78=0.78*990+0.78=0.78*9917.8*99+0.78=7.8*100-7.8+0.78=780-7.02=772.98
=7.8×（100-1）+0.78=78-7.8+0.78=70.98
7.8*(100-1)+7.8*0.1=780-7.02=772.98
7.8（99+0.1）
7.8乘99+1的和
