Problem:
Consider the arithmetic sequence defined by $a_1=\frac{69}{35}$, and $a_n=a_{n-1}+-3 \pi$ for $n > 1$. Compute the nth partial sum, $S_n=\sum_{k=1}^n a_k$, where $n=27$.
Answer:
$\frac{27}{2} \left(\frac{138}{35}-78 \pi \right)$
{'source': 'AMPS', 'file': 'amps/mathematica/algebra/arithmetic_series/42677.txt'}