The first Common Ratio : r = 2
Explicit formula : a(n) = - 2 ( 2 )^n-1
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The second Common Ratio : r = 1/3
Explicit formula : a(n) = 27 ( 1/3l)^n-1
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The third Commom Ratio : r = - 3
Explicit formula : - 5 ( - 3 )^n-1
