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rd
9, 9 + d, 9 + 2d
9, 11 + d, 29 + 2d
. d = 9r − 11 . 9r = 11 + d
. 9r 2 = 29 + 2d = 29 + 2(9r − 11) = 7 + 18r
. 9r2 − 18r − 7 = 0
9r 2 − 18r − 7 = 0 ⇔ (3r + 1)(3r − 7) = 0
⇔ r = −1 r =7
3 3
. 9r2 = 9 − 1 2 = 1 r = −1
3 3
. 9r2 = 9 7 2 = 49 r =7
3 3
.1
(Series) (.)