JEE Main & Advanced Mathematics Statistics Question Bank Critical Thinking

  • question_answer
    The mean of n items is \[\bar{x}\]. If the first term is increased by 1, second by 2 and so on, then new mean is            [DCE 1998]

    A)                 \[\bar{x}+n\]          

    B)                 \[\bar{x}+\frac{n}{2}\]

    C)                 \[\bar{x}+\frac{n+1}{2}\]      

    D)                 None of these

    Correct Answer: C

    Solution :

                    Let \[{{x}_{1}},{{x}_{2}},\]....... \[{{x}_{n}}\] be n items. Then, \[\bar{x}=\frac{1}{n}\Sigma {{x}_{i}}\]                                 Let \[{{y}_{1}}={{x}_{1}}+1,\ {{y}_{2}}={{x}_{2}}+2,\ {{y}_{3}}={{x}_{3}}+3,..,{{y}_{n}}={{x}_{n}}+n\]                                 Then the mean of the new series is \[\frac{1}{n}\Sigma {{y}_{i}}=\frac{1}{n}\sum\limits_{i=1}^{n}{({{x}_{i}}+i)}\]                                                 \[=\frac{1}{n}\sum\limits_{i=1}^{n}{{{x}_{i}}}+\frac{1}{n}(1+2+3+.....+n)\]                                 \[=\bar{x}+\frac{1}{n}.\frac{n(n+1)}{2}\]\[=\bar{x}+\frac{n+1}{2}\].


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