3/6=1/2 3/6与1/2
7/8 7/8与14/16
15/20=3/4 15/20与3/4
40/120=1/3 40/120与1/3
3/7 3/7与9/21
3/4
1/2
1/3
9/21=3/7
14/16=7/8