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Câu hỏi: Đặt $\alpha ={{\log }_{2}}20$. Khi đó ${{\log }_{20}}5$ bằng:
A. $\dfrac{\alpha -3}{\alpha }$
B. $\dfrac{\alpha -1}{\alpha }$.
C. $\dfrac{\alpha -2}{\alpha }$.
D. $\dfrac{\alpha -4}{\alpha }$.
A. $\dfrac{\alpha -3}{\alpha }$
B. $\dfrac{\alpha -1}{\alpha }$.
C. $\dfrac{\alpha -2}{\alpha }$.
D. $\dfrac{\alpha -4}{\alpha }$.
Ta có ${{\log }_{20}}5=\dfrac{{{\log }_{2}}5}{{{\log }_{2}}20}=\dfrac{{{\log }_{2}}\dfrac{20}{4}}{{{\log }_{2}}20}=\dfrac{{{\log }_{2}}20-{{\log }_{2}}4}{{{\log }_{2}}20}=\dfrac{\alpha -2}{\alpha }$
Đáp án C.