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

  • question_answer
    The series\[3\log 2+\frac{1}{4}-\frac{1}{2}{{\left( \frac{1}{4} \right)}^{2}}+\frac{1}{3}{{\left( \frac{1}{4} \right)}^{3}}+....\] is equal to:

    A)  \[log5\]                        

    B)  \[log2\]          

    C)  \[log6\]                        

    D)  \[\log 10\]

    Correct Answer: D

    Solution :

    [d] \[\because 3\log 2+\frac{1}{4}-\frac{1}{2}{{\left( \frac{1}{4} \right)}^{2}}+\frac{1}{3}.{{\left( \frac{1}{4} \right)}^{3}}+......\] \[=3\log 2+\log \left( 1+\frac{1}{4} \right)\]             \[\left[ \because \log (x+1)=x-\frac{{{x}^{2}}}{2}+\frac{{{x}^{3}}}{3}+......\infty  \right]\]             \[=3\log 2+\log \left( \frac{5}{4} \right)=3\log 2+\log 5-2\log 2\] \[=31og2+log5=\log 10\] Hence option [d] is correct.


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