Geometric Series Practice

1. Find the sum.
   

   $\displaystyle\sum\limits_{k=1}^{40}{{(25 - 2k)}}=$

2. An arithmetic sequence is defined as follows:

   $\begin{cases}a_1 = -138\\a_i = a_{i - 1} + 6\end{cases}$
   

   Find the sum of the first ‍35 terms in the sequence.

3. Find the sum.
   
   $-111 + (-102) + (-93) + \dots + 159 + 168 =$

4. Find the sum.
   
   $\displaystyle\sum\limits_{k=1}^{50}{{(131-4k)}}=$

Source: Khan Academy, https://www.khanacademy.org/math/precalculus/x9e81a4f98389efdf:series/x9e81a4f98389efdf:arith-series/e/arithmetic_series
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