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S9 = 414
n/2 . (2a + (n - 1) . b) = 414
9/2 . (2 . 26 + (9 - 1) . b) = 414
9/2 . (52 + 8b) = 414
234 + 36b = 414
36b = 414 - 234
36b = 180
b = 180 : 36
b = 5