Sum of arithmetic series

  • Thread starter Blister
  • Start date
1. The problem statement, all variables and given/known data

The sum of the first 4 terms in an arithmetic series is -8 and the sum of the first 5 terms is 85. Determine the first term and the common difference.

2. Relevant equations

tn = a + (n-1)d
Sn = n/2 (2a + (n-1)d)

3. The attempt at a solution
 

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