4.9×90+4.9＝4.9×（90+1）＝4.9×91＝（5-0.1）×91＝5×91-0.1×91＝455-9.1＝445.9
4.9×90+4.9=4.9×（100－10）+5-0.1=490－49+5－0.1=490-50+6－0.1=445.9
