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Câu hỏi: Với $a>0$ biểu thức ${{\log }_{5}}\left( \dfrac{{{a}^{2}}}{25} \right)$ bằng
A. $2\left( {{\log }_{5}}a+1 \right).$
B. $2+{{\log }_{5}}a.$
C. $2\left( {{\log }_{5}}a-1 \right).$
D. $\dfrac{1}{2}{{\log }_{5}}a.$
A. $2\left( {{\log }_{5}}a+1 \right).$
B. $2+{{\log }_{5}}a.$
C. $2\left( {{\log }_{5}}a-1 \right).$
D. $\dfrac{1}{2}{{\log }_{5}}a.$
Ta có: ${{\log }_{5}}\left( \dfrac{{{a}^{2}}}{25} \right)={{\log }_{5}}{{a}^{2}}-{{\log }_{5}}{{5}^{2}}=2\left( {{\log }_{5}}a-1 \right).$
Đáp án C.