Find sn for the given arithmetic series.
a1 = 18, d = –3, n = 14

The sequence is given recursively by

By subsitution, we can solve for in terms of :

and so on, with

Then the sum of the first 14 terms of the sequence is

- 21

Step-by-step explanation:

The sum to n terms of an arithmetic sequence is

= [ 2a₁ + (n - 1)d ], thus

= [( 2 × 18) + (13 × - 3) ]

= 7 (36 - 39) = 7 × - 3 = - 21

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