Sum the following series to `n` terms: `5+7+13+31+85+`

Explanation

Difference between terms in GP

Method of difference

`S_n=5+7+13+31+85+...+T_n-(1)`

`S_n=0+5+7+13+31+81+...+T_n-(2)`

Subtracting equation 2 from 1

`0=5+2+6+18+...+(T_n-T_(n-1))-T_n`

`T_n=5+2(3^(n-1)-1)/(3-1)`

`T_n=5+(3^(n-1)-1)`

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