14×5/13
=(13+1)×5/13
=13×5/13+5/13×1
=5+5/13
=5又5/13
18/19×18
=18/19×(19-1)
=18/19×19-18/19×1
=18-18/19
=17又1/19
14×5/13
=(13+1)*5/13
=13*5/13+5/13*1
=5+5/13
=5又5/13
18/19×18
=18/19*(19-1)
=18/19*19-18/19*1
=18-18/19
=17又1/19
凑数而已
=20-(1 2 3。。。 19)=20-(1 19)*19/2=20-20*19/2=20-190=-170 20-19-18-17-16-15-14-13-12-11-10-9-8-7-6-5-4-3-