11th Class Mathematics Sample Paper Mathematics Olympiad - Sample Paper-7

  • question_answer
    The sum of the series \[lo{{g}_{4}}^{2}-lo{{g}_{8}}^{2}+lo{{g}_{10}}^{2}.....\]up to \[\infty \]is:

    A)   \[lo{{g}_{e}}2-1\]                   

    B)  \[1-lo{{g}_{e}}2\]      

    C)  \[1+lo{{g}_{e}}2\]      

    D)  e

    Correct Answer: B

    Solution :

    [b] \[\because lo{{g}_{4}}^{2}-lo{{g}_{^{8}}}^{2}-lo{{g}_{16}}^{2}+.....up\,to\,\infty \] \[=\frac{1}{{{\log }_{1}}^{4}}-\frac{1}{{{\log }_{2}}^{8}}+\frac{1}{{{\log }_{2}}^{16}}+.....up\,to\,\infty \] \[=\frac{1}{4}-\frac{1}{3}+\frac{1}{4}+....up\,to\,\infty \] \[=1-1+\frac{1}{2}-\frac{1}{3}+\frac{1}{4}+.....up\,to\,\infty \] \[=1-\left( 1-\frac{1}{2}-\frac{1}{3}+\frac{1}{4} \right)+......up\,to\,\infty \] \[=1-log\left( 1+1 \right)=1-log2.\] Hence, option [b] is correct.


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