JCECE Engineering JCECE Engineering Solved Paper-2012

  • question_answer
    Find the sum of the series\[\frac{1}{2\cdot 3}+\frac{1}{4\cdot 5}+\frac{1}{6\cdot 7}+...\]

    A) \[\log \frac{e}{2}\]                         

    B) \[\log \frac{e}{4}\]

    C) \[\log \frac{2}{3}\]                          

    D) \[\log \frac{2}{4}\]

    Correct Answer: A

    Solution :

    \[\frac{1}{2\cdot 3}+\frac{1}{4\cdot 5}+\frac{1}{6\cdot 7}+...\]                 \[=\frac{1}{2}-\frac{1}{3}+\frac{1}{4}-\frac{1}{5}+\frac{1}{6}-\frac{1}{7}+...\]                 \[=1-1+\frac{1}{2}-\frac{1}{3}+\frac{1}{4}-\frac{1}{5}+...\]                 \[=1-\left( 1-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+\frac{1}{5}+... \right)\]                 \[=1-\log 2=\log e-\log 2\]                 \[=\log \frac{e}{2}\]


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