Question 352188
{{{a[1]=1}}}
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{{{a[n+1]=a[n]+(n+1)}}}
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{{{a[2]=a[1]+2=1+2=3}}}
{{{a[3]=a[2]+3=3+3=6}}}
{{{a[4]=a[3]+4=6+4=10}}}
{{{a[5]=a[4]+5=10+5=15}}}
{{{a[6]=a[5]+6=15+6=21}}}