1972 AHSME Problems/Problem 19

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Solution

We begin by evaluating the sum of the first $n$ terms of the sequence with $1$, $2$, and $3$. We find that the sum of the first term of the sequence is $1$, the sum of the first two terms is $4$, and the sum of the first three terms $11$. From here, we can simply guess and check the answer choices and will find that only $\boxed{D}$ works.

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