1)
a=0,373737...
100a = 37,373737...
100a = 37 + 0,373737...
100a = 37 + a
99a = 37
a = 37/99
-----
2)
b=0,123123123...
1000b = 123,123123123...
1000b = 123 + 0,123123123...
1000b = 123 + b
999b = 123
b = 123/999
b = 41/333
-----
3)
12,090909... = 12 + 0,090909...
n = 0,09090909...
N = 10n = 0,9090909...
100N = 90,909090...
100N = 90 + 0,909090...
100N = 90 + N
99N = 90
N = 90/99
N = 10/11
n=N/10
n = 1/11
12,09090909... = 12 + (1/11)
12,09090909... = (132/11) + (1/11)
12,09090909... = 133/11
-----
Sauf distraction.