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Câu hỏi: Nếu ${{\log }_{2}}x=5{{\log }_{2}}a+4{{\log }_{2}}b$ ( $a,b>0$ ) thì $x$ bằng.
A. ${{a}^{4}}{{b}^{5}}$.
B. $5a+4b$.
C. $4a+5b$.
D. ${{a}^{5}}{{b}^{4}}$.
Ta có ${{\log }_{2}}x=5{{\log }_{2}}a+4{{\log }_{2}}b\Leftrightarrow {{\log }_{2}}x={{\log }_{2}}{{a}^{5}}{{b}^{4}}\Leftrightarrow x={{a}^{5}}{{b}^{4}}$.
A. ${{a}^{4}}{{b}^{5}}$.
B. $5a+4b$.
C. $4a+5b$.
D. ${{a}^{5}}{{b}^{4}}$.
Ta có ${{\log }_{2}}x=5{{\log }_{2}}a+4{{\log }_{2}}b\Leftrightarrow {{\log }_{2}}x={{\log }_{2}}{{a}^{5}}{{b}^{4}}\Leftrightarrow x={{a}^{5}}{{b}^{4}}$.
Đáp án D.