1/2,1,1/3,4/3,( 1/4 ),( 2 )
规律:1/2,1(1/2*2)
1/3,4/3 (1/3*4)
1/4,2(1/4*8)
1/2 * 2 = 1
1 / 3= 1/3
1/3 * 4= 4/3
4/3 / 5 =4/15
4/15 * 6=8/5
8/5 / 7= 8/35
规律: 1/2,1(1/2*2)
1/3,4/3 (1/3*4)
1/4,2(1/4*8)