Question 214359
{{{s = (n/2)(2a+(n-1)d)}}} Multiply both sides by {{{2/n}}}
{{{2s/n = 2a+(n-1)d}}} Subtract 2a from both sides.
{{{(2s/n)-2a = (n-1)d}}} Simplify the left side.
{{{(2s-2an)/n = (n-1)d}}} Divide both sides by {{{(n-1)}}}
{{{(2s-2an)/n(n-1) = d}}} Simplify the left side.
{{{d = 2(s-an)/(n^2-n)}}}