7.8x10.1=7.8x(10+0.1)=7.8x10+7.8x0.1=78.78
7.8x10.1=7.8x10+7.8x0.1=78+0.78=78.78
=7.8x（10+0.1）=78+0.78=78.78
= 7.8×10 + 7.8×0.1= 78+0.78= 78.78
