Question 624868
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Hi,
John saves Nu. 2 in january, Nu. 6 in february, Nu. 18 in march and so on.
 {{{a[n] = a[1]*r^(n-1)}}} , r = 3
 {{{a[n] = 2*3^(n-1)}}}
{{{S[n] =  a[1]((1-r^n)/(1-r))}}}
{{{S[12] =  2((1-3^n)/(-2))= 531440}}}