8 целых 7/12-2целых17/26=6целых4/26=6 целых2/13
Имеем k членов арифметической прогрессии с шагом 3. k-тый член прогрессии ak = a1+(k-1)*3=59+3*k-3=56+3*k=6*n-1 => k = (6*n-57)/3
S k членов арифм.прогрессии = (59 + 6*n-1)*k/2=(58+6*n)*(6*n-57)/6=6*n*n+n-551
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