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Câu hỏi: Với mọi số thực $a$ dương, $3{{\log }_{3}}\dfrac{a}{3}$ bằng
A. $9\left( {{\log }_{3}}a-1 \right).$
B. $-\left( {{\log }_{3}}a-1 \right).$
C. $3\left( {{\log }_{3}}a-1 \right).$
D. ${{\log }_{3}}a.$
A. $9\left( {{\log }_{3}}a-1 \right).$
B. $-\left( {{\log }_{3}}a-1 \right).$
C. $3\left( {{\log }_{3}}a-1 \right).$
D. ${{\log }_{3}}a.$
Ta có $3{{\log }_{3}}\dfrac{a}{3}=3\left( {{\log }_{3}}a-{{\log }_{3}}3 \right)=3\left( {{\log }_{3}}a-1 \right).$
$x$ | $-\infty $ | $-1$ | $0$ | $1$ | $2$ | $+\infty $ | |||||
$y'$ | $+$ | $0$ | $-$ | $0$ | $+$ | $ \left\| {} \right.$ | $-$ | $0$ | $+$ |
Đáp án C.