Question 374462
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Hi,
for an arithmetic series: {{{S[n]=(n/2)(2*a[1]+ (n-1)d)}}}
In this series d = 12
{{{S[n]=(n/2)(2*a[1]+ (n-1)12)}}}
question states:
{{{S[7]=(7/2)(2*a[1]+ (6)*12)= 287}}}
{{{(7/2)2*a[1]+ (7/2)(6)*12= 287}}}
{{{7*a[1]+ 252 = 287}}}
  7* a[1] = 287-252  
  7* a[1] = 35
     a[1] = 5