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1. U12 = 122
a + 11b = 122
23 + 11b = 122
11b = 122 - 23
11b = 99
b = 99 : 11
b = 9
2. S15 = n/2 . (2a + (n - 1) . b)
S15 = 15/2 . (2 . 23 + (15 - 1) . 9)
S15 = 15/2 . (46 + 14 . 9)
S15 = 15/2 . (46 + 126)
S15 = 15/2 . 172
S15 = 1290