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ACT® Math

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Sum of a Sequence

ACTMAT-PS1ETV

If S represents the sum of the first 6 terms of the sequence:

$$ a+b, 2a+{ b }^{ 2 }, 3a+{ b }^{ 3 },... $$

...then which of the following is S?

A

$ 21a+\cfrac { b-{ b }^{ 7 } }{ 1-b } $

B

$ 21a+\cfrac { 1-{ b }^{ 8 } }{ 1+b } $

C

$ 21a+\cfrac { b-{ b }^{ 8 } }{ 1-b } $

D

$ 28a+\cfrac { 1-{ b }^{ 7 } }{ 1-b } $

E

$ 21a+\cfrac { b-{ b }^{ 8 } }{ 1+b } $