Question 1146193
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If the sum of the first and third terms is 0, then the second term is 0; and the first and third terms are -d and d, where d is the common difference.<br>
Then the sum of all 9 terms is the sum of the 4th through 9th terms; those terms are 2d, 3d, ..., and 7d.  So<br>
{{{2d+3d+4d+5d+6d+7d = 81}}}<br>
Solve that to find d to complete the problem.