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[tex]\displaystyle Barisan \ Geometri \ [XII] \\ \\ U_1 \ = \ \frac{1}{a^2} \ \xrightarrow \ a \ \ \textgreater \ \ 0 \\ U_2 \ = \ a^k \\ U_{10} \ = \ a^{70} \\ \\ U_{10} \ = \ a^{70} \\ a.r^9 \ = \ a^{70} \\ \frac{1}{a^2}.r \ = \ a^{70} \\ r^9 \ = \ a^{72} \\ r \ = \ a^8 \\ \\ U_2 \ = \ a^k \\ a.r \ = \ a^k \\ \frac{1}{a^2}.a^8 \ = \ a^k \\ a^6 \ = \ a^k \\ k \ = \ 6 \ \xrightarrow \ Answer [/tex]