Question 465630
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Hi
3+9+15+21+...+57 
arithmetic series, d= 6
{{{a[n]=a[1]+(n-1)*d}}}
   57 = 3 + (n-1)6
   60 = 6n
    n = 10, 57 is the 10th term
{{{S[n]=(n/2)(2*a[1]+ (n-1)d)}}}
{{{S[10]=(10/2)(2*3+ (9)6)}}}
{{{S[10]=5*60 = 300 )}}}