9th Class Mathematics Related to Competitive Exam Question Bank Logarithms

  • question_answer
    If \[lo{{g}_{a}}(ab)=x,\]then \[lo{{g}_{b}}(ab)\]is:

    A)  \[\frac{1}{x}\]                         

    B)  \[\frac{x}{1+x}\]

    C)  \[\frac{x}{1-x}\]                       

    D)  \[\frac{x}{x-1}\]

    Correct Answer: D

    Solution :

    (d): \[lo{{g}_{a}}(ab)=x\Leftrightarrow \frac{\log ab}{\log a}=x\] \[\Leftrightarrow \frac{\log a+\log b}{\log a}=x\] \[\Leftrightarrow 1+\frac{\log b}{\log a}=x\Leftrightarrow \frac{\log b}{\log a}=x-1\] \[\Leftrightarrow \frac{\log b}{\log a}=\frac{1}{x-1}x\Leftrightarrow 1+\frac{\log a}{\log b}=1+\frac{1}{x-1}\] \[\Leftrightarrow \frac{\log b}{\log b}+\frac{\log a}{\log b}=\frac{x}{x-1}\Leftrightarrow \frac{\log b+\log a}{\log b}=\frac{x}{x-1}\] \[\Leftrightarrow \frac{\log \left( ab \right)}{\log b}=\frac{x}{x-1}\Leftrightarrow {{\log }_{b}}(ab)=\frac{x}{x-1}\]


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