Question 854831
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Hi,
3+10+17+24...  d = 7  {{{a[n] = 3 + (n-1)7 = 7n - 4}}}
{{{sum(a[n], n=1,18 )= sum(7n -4, n=1, 18) }}}  18 rows, 7n-8 beads in each row
{{{a[18] = 7*18 - 4 = 119 }}}
{{{sum(7n -4, n=1, 18 )= (n(a[1]+a[18]))/2)}}} = 9(3+119) = 1098,the total number of beads in the necklace