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b = U2 - U1
b = 3 - 1
b = 2
Un = 149
a + (n - 1) . b = 149
1 + (n - 1) . 2 = 149
1 + 2n - 2 = 149
2n - 1 = 149
2n = 149 + 1
2n = 150
n = 150 : 2
n = 25
S25 = n
n/2 . (a + Un) = n
25/2 . (1 + 149) = n
25/2 . 150 = n
1875 = n