7.8*9.9=7.8*(10-0.1)=7.8*10-7.8*0.1=78-0.78=77.22
9.9*7.8=（10-0.1）*7.8=78-0.78=77.22
8.7*9.9=8.7x(10-0.1)=8.7x10-8.7x0.1=87-0.87=86.13祝学习进步，望采纳~~不懂的还有追问
8.7*9.9=8.7(10-0.1)=87-0.87=86.13
