13.4 Exercise 4

Find the sum of the first n terms of an arithmetic sequence.

1. ⓐ Find the sum of the first 30 terms of the arithmetic sequence: 7, 10, 13, 13, 19,...
2. ⓑ Find the sum of the first 50 terms of the arithmetic sequence whose general term is $a_{n} = 2 n - 5$ .
3. ⓒ Find the sum
   $\sum_{i = 1}^{30} \left(6 i - 4\right)$