4.9*10.1＝4.9*（10+0.1）＝4.9*10+4.9*0.1＝49+0.49＝49.49
=(5-0.1)*(10+0.1)=5*10-0.1*10+5*0.1-0.1*0.1=50-1+0.5-0.01=49.49
