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Un =a + (n-1)b
161 = 7 + (n-1)7
154 = 7n - 7
161 = 7n
n = 23
S23 = 23/2 ( a + Un)
S23 = 23/2 (7 + 23)
S23 = 23/2 × 30
S23 = 345