Question 976138
{{{system(a+(3-1)d=-12,a+(7-1)d=8)}}}
{{{system(a+2d=-12,a+6d=8)}}}
{{{system(a=-2d-12,a=-6d+8)}}}
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{{{-2d-12=-6d+8}}}
{{{d+6=3d-4}}}
{{{6+4=2d}}}
{{{10=2d}}}
{{{highlight_green(d=5)}}}
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{{{a=-2d-12}}}
{{{a=-2*5-12}}}
{{{a=-10-12}}}
{{{highlight_green(a=-22)}}}


General term for the sequence is  {{{highlight(-22+(n-1)*5)}}}.


You did not say what limits for the sum you want.  You can lookup the formula for sum of an arithmetic sequence or you can derive it yourself.  A crude way to show the formula is like,


{{{(n/2)(FirstTerm+LastTerm)}}}