Sum of arithmetic series sample problem with solution

Problem 1: 

Find n and sn for the series 14, 11, 18, 5, ... -82.

Solution: 

Using the nth term of the arithmetic sequence formula, 

Where a_n is the nth term

a_1 is the first term

n is the number of terms

d is the common difference

a_n=-82

a_1=14

d=-3 ( because the succeeding is 3 less than the preceding term.)

Solving for n:

Solving for sum using the sum formula,