Question 1070092
{{{d=8}}}
{{{a[1]=9}}}
.
.
{{{S[n]=(n/2)(2*a[1]+(n-1)d)}}}
Substituting,
{{{(n/2)(2*9+(n-1)8)=636}}}
{{{n(18+8n-8)=1272}}}
{{{(8n+10)n=1272}}}
{{{(4n+5)n=636}}}
{{{4n^2+5n-636=0}}}
{{{(n-12)(4n+53)=0}}}
We're only interested in the integer solution.
{{{n-12=0}}}
{{{n=12}}}