Question 607821
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Hi
Geomtric sequence: {{{a[n] = a[1]*r^(n-1)}}}
{7+(-21)+63+(-189)+...}
Note the Change of sign:
{{{a[n] = 7*(-3)^(n-1)}}}
{{{a[18] = 7*(-3)^(17)= highlight(903981141)}}}
  {{{sum( a[i], i=1, n ) = a[1]((1-r^n)/(1-r))= (7/4)(1-r^n) = 3829}}}
 7/4(1-r^n) = 3829
  (1-(-)3^n) = (4/7)3829 = 2188
     3^n = 2187
       n = 7