gaokao 2020 Q17
12 marks
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Let the geometric sequence $\left\{ a _ { n } \right\}$ satisfy $a _ { 1 } + a _ { 2 } = 4 , a _ { 3 } - a _ { 1 } = 8$ .
(1) Find the general term formula for $\left\{ a _ { n } \right\}$;
(2) Let $S _ { n }$ be the sum of the first $n$ terms of the sequence $\left\{ \log _ { 3 } a _ { n } \right\}$. If $S _ { m } + S _ { m + 1 } = S _ { m + 3 }$, find $m$ .