Question 1120044
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s=1/2d(a+1)
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Not clear as written.  If you want "a", then just undo what was done to it.



Written as you did, may mean  {{{s=1/2d(a+1)}}}.


{{{s(a+1)=1/(2d)}}}


{{{a+1=1/(2ds)}}}


{{{a=1/(2ds)-1}}}