Find a Formula for the Sum of the First N $9,12,15,18,21 , \ldots$

question 47

Multiple Choice

Find a formula for the sum of the first n terms of the sequence.​ $9,12,15,18,21 , \ldots$ ​

• A) \(S _ { n } = \frac { n } { 2 } ( 3 n + 15 )\)
• B) \(S _ { n } = \frac { n } { 2 } ( 3 n + 3 )\)
• C) \(S _ { n } = \frac { n } { 2 } ( - 3 n - 15 )\)
• D) \(S _ { n } = \frac { n } { 2 } ( 3 n - 15 )\)
• E) \(S _ { n } = \frac { n } { 2 } ( - 3 n + 15 )\)

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