11th Class Mathematics Sample Paper Mathematics Olympiad - Sample Paper-8

  • question_answer
    The sum of the series \[\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{3.4}......\] up to \[\infty \] is equal to.

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

    B) \[lo{{g}_{e}}2-1\]      

    C)  \[2.lo{{g}_{e}}2\]       

    D)  \[2.lo{{g}_{e}}2-1\]

    Correct Answer: D

    Solution :

    \[\because \frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{3.4}+.....to\,\infty .\] \[=\frac{2-1}{1.2}-\frac{3-2}{2.3}+\frac{4-3}{3.4}+.......up\,to\,\infty .\] \[=1-\frac{1}{2}-\frac{1}{2}+\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+.......up\,to\,\infty \] \[=\frac{2}{2}-\frac{2}{2}+\frac{2}{3}-\frac{1}{4}+\frac{2}{5}+.......up\,to\,\infty \] \[=1-2\left( \frac{1}{2}-\frac{1}{3}+-\frac{1}{4}++.....up\,to\,\infty  \right)\] \[=1-21o{{g}_{e}}2\]


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