Sunt doua siruri care reprezinta progresii aritmetica:
a(n)=3n+2/4, cu ratia r= 3 si termeni 3+2/4, 6+2/4, 9+2/4,... si
b(n)=5- 4n/3 care are termenii:5, 5-4/3, 5-8/3, 5-12/3, etc.............. ratia r= -4/3.
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