Question 501518
A general formula for this would be
{{{ (-1)^n * 3^n }}} where {{{n}}} varies from
{{{0}}} to any positive integer
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The ratio of the nineteenth term to the twentieth term:
{{{ ((-1)^18*3^18) / ((-1)^19*3^19) = 1 / (( -1)*3) }}}
{{{  1 / (( -1)*3) = -(1/3) }}}
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Note that I start counting terms with {{{0}}},  so the 
the 19th term has {{{ n = 18 }}}
the 20th term has {{{ n = 19 }}}