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If the sum of the first n terms of the series 4 + 7 + 10 + ... is 209, find n.

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n/2 (2*4 + (n-1)*3) = 209
1/2 (3n^2+5n) = 209
3n^2+5n-418 = 0
(3n+38)(n-11) = 0
n=11

209

Very helpful and simplify. Thanks

The answer is 11.

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