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3 + 5d = 12

5d = 9

d = 9 = 1.8
5

. 3, 4.8, 6.6, 8.4, 10.2, 12

[Geometric Sequences] (.)

.(common ratio) {an }

r an +1 = anr

.n

4, 12, 36, 108, …

.3

4, − 12, 36, − 108, …

.−3

.c a b =a cba
cb
b2 = ac ac

b = ± ac {an }

(geometric mean)

r

a1, a1r, a1r 2, a1r 3, …, a1r n−1, …