Question 392757
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Hi
 geometric sequence { 2, - 1, 1/2...} r = -(1/2)
{{{a[n] = a[1]*r^(n-1)}}}
Note: {{{a[3] = 2*r(-1/2)^2}}} = 2/4 = 1/2

{{{a[5] = 2*(-1/2)^(4) = 2/16 }}} =  1/8