简便算9/4-1/4-1/5=1-1/5=4/5
9/4-(1/4+1/5)＝9/4-（5/20+4/20）＝45/20-9/20＝36/20＝9/5
=9/4-1/4-1/5=8/4-1/5=2-1/5=9/5=1.8
=9/4-(5/20+4/20)=45/20-9/20=36/20=9/5
9/4－1/4－1/5=1+4/5
