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Câu hỏi: Với $a$ là số thực dương tuỳ ý khác $1,\text{lo}{{\text{g}}_{a}}2a$ bằng
A. $1+\text{lo}{{\text{g}}_{2}}a$.
B. $1-\text{lo}{{\text{g}}_{a}}2$.
C. $1+\dfrac{1}{\text{lo}{{\text{g}}_{2}}a}$.
D. $2$.
A. $1+\text{lo}{{\text{g}}_{2}}a$.
B. $1-\text{lo}{{\text{g}}_{a}}2$.
C. $1+\dfrac{1}{\text{lo}{{\text{g}}_{2}}a}$.
D. $2$.
Ta có $\text{lo}{{\text{g}}_{a}}2a=\text{lo}{{\text{g}}_{a}}2+\text{lo}{{\text{g}}_{a}}a=1+\text{lo}{{\text{g}}_{a}}2=1+\dfrac{1}{\text{lo}{{\text{g}}_{2}}a}$.
Đáp án C.