Question 1000972
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- Write the formula for the sum of an arithmetic progression;


- substitute numerical values of  {{{a[1]}}}  and  {{{d}}},  the common difference;

- replace  n, the number of the terms,  by  x;


- you will get the quadratic parabola function of  x;


- find  x0  where the maximum of this quadratic function is achieved  (x-coordinate of the vertex);


- take  n  closest to  x0.