8th Class Mathematics Related to Competitive Exam Question Bank Logarithms

  • question_answer
    If \[\mathbf{lo}{{\mathbf{g}}_{\mathbf{10}}}\mathbf{5+lo}{{\mathbf{g}}_{\mathbf{10}}}\left( \mathbf{5x+1} \right)\mathbf{=lo}{{\mathbf{g}}_{\mathbf{10}}}\left( \mathbf{x+5} \right)\mathbf{+1,}\]then x is equal to:

    A)  1                                

    B)  3

    C)  5                                

    D)  10

    Correct Answer: B

    Solution :

    (b): \[lo{{g}_{10}}5+lo{{g}_{10}}(5x+1)=lo{{g}_{10}}(x+5)+1\] \[\Rightarrow lo{{g}_{10}}5+lo{{g}_{10}}(5x+1)=lo{{g}_{10}}\left[ (x+5) \right]+{{\log }_{10}}10\] \[\Rightarrow lo{{g}_{10}}5+lo{{g}_{10}}(5x+1)=lo{{g}_{10}}\left[ 10(x+5) \right]\] \[\Rightarrow 5(5x+1)=10(x+5)\] \[\Rightarrow 5x+1=2x+10\Rightarrow \,\,\,\,\,3x=9\,\,\,\,\Rightarrow \,\,\,\,x=3.\]


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