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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?

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

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

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

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

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