7.8×0点99+9点九=7.8×0.99+0.99×10=0.99×(7.8+10)=(1-0.01)×17.8=17.8-0.178=17.622。
7.8×0.99+9.9=9.9×(0.78+1)=9.9×1.78=(10-0.1)×1.78=17.8-0.178=17.622
=9.9X(0.78十1)=9.9X1.78=10X1.78一0.1X1.78=17·8一0.178=17.622