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a1 + 5d = 21
2a1 + 16d = 0

a1 + 5d = 21
a1 + 8d = 0

−3d = 21
d = −7
a1 + 8d = 0

.a`1 = −8d = −8 × (−7) = 56
.a3 = a1 + 2d = 56 + 2 × (−7) = 42

(Geometric Series) (.)

.

1 + 2 + 4 + 8 + 16 + + 128

1, 2, 4, 8, 16, …, 128

. r a1, a2, …, an

a1, a1r, a1r 2, …, a1r n−1

Sn = a1 + a1r + a1r 2 + + a1r n−1