Question 979587


{{{a[1]}}} = 3 

{{{a[2]}}} = 6 

{{{a[3]}}} = 10 

{{{a[4]}}} = 15 

{{{a[5]}}} = 21


General formula:  {{{a[k]}}} = {{{a[k-1]}}} + {{{(k+1)}}}, 
for  k = 2, 3, 4, 5.