1, 6, 11,...
Это арифметический прогрессия. a1 = 1, d = 5.
a20 = a1 + 19d = 1 + 19*5 = 96
S(20) = (a1 + a20)*20/2 = (1 + 96)*10 = 970
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1, 6, 11,...
Это арифметический прогрессия. a1 = 1, d = 5.
a20 = a1 + 19d = 1 + 19*5 = 96
S(20) = (a1 + a20)*20/2 = (1 + 96)*10 = 970