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Câu hỏi: Cho $f\left( x \right)={{3}^{x}}$ thì $f\left( x+3 \right)-f\left( x \right)$ bằng
A. $28 .$
B. $189 .$
C. $28f\left( x \right) .$
D. $26f\left( x \right).$
Ta có $f\left( x+3 \right)-f\left( x \right) $ $= {{3}^{x+3}}-{{3}^{x}}$ $={{3}^{x}}\left( {{3}^{3}}-1 \right)$ $=26f\left( x \right)$.
A. $28 .$
B. $189 .$
C. $28f\left( x \right) .$
D. $26f\left( x \right).$
Ta có $f\left( x+3 \right)-f\left( x \right) $ $= {{3}^{x+3}}-{{3}^{x}}$ $={{3}^{x}}\left( {{3}^{3}}-1 \right)$ $=26f\left( x \right)$.
Đáp án D.