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Câu hỏi: Đặt ${{\log }_{2}}9=a$, khi đó ${{\log }_{3}}18$ bằng.
A. $\dfrac{2-2\text{a}}{a}$
B. $\dfrac{a}{2+2\text{a}}$
C. $\dfrac{a}{1-a}$
D. $\dfrac{2\text{a}+2}{a}$
A. $\dfrac{2-2\text{a}}{a}$
B. $\dfrac{a}{2+2\text{a}}$
C. $\dfrac{a}{1-a}$
D. $\dfrac{2\text{a}+2}{a}$
Ta có ${{\log }_{3}}18={{\log }_{3}}2+2=\dfrac{1}{{{\log }_{2}}3}+2=\dfrac{2}{{{\log }_{2}}9}+2=\dfrac{2}{a}+2=\dfrac{2\text{a}+2}{a}$.
Đáp án D.