Question 323636
{{{S=(n(A1+An)/2)=3600}}}
{{{n(1+An)=7200}}}
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{{{An=A1+(n-1)*2}}}
{{{An=1+2n-2}}}
{{{An=2n-1}}}
Substituting,
{{{n(1+2n-1)=7200}}}
{{{2n^2=7200}}}
{{{n^2=3600}}}
{{{n=60}}}
{{{A(60)=2(60)-1=119}}}