Question 851217
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Hi, 
{3+10+17+24+.... }  Arithmetic Series d = 7
{{{sum( a[i], i=1, n ) = (n/2)(2*a[1]+ (n-1)d)}}}
{{{sum( a[i], i=1, 18 ) = (18/2)(2*3 + (17)7) = highlight(1125)}}}