JEE Main & Advanced Mathematics Statistics Question Bank Critical Thinking

  • question_answer
    The S.D. of a variate x is s. The S.D. of the variate \[\frac{ax+b}{c}\] where a, b, c are constant, is                                [Pb. CET 1996]

    A)                 \[\left( \frac{a}{c} \right)\,\sigma \]      

    B)                 \[\left| \frac{a}{c} \right|\,\sigma \]

    C)                 \[\left( \frac{{{a}^{2}}}{{{c}^{2}}} \right)\,\sigma \]         

    D)                 None of these

    Correct Answer: B

    Solution :

                    Let \[y=\frac{ax+b}{c}\] i.e., \[y=\frac{a}{c}x+\frac{b}{c}\]                                 i.e., \[y=Ax+B\], where \[A=\frac{a}{c}\],\[B=\frac{b}{c}\]             \ \[\bar{y}=A\bar{x}+B\]             \ \[y-\bar{y}=A(x-\bar{x})\] Þ \[{{(y-\bar{y})}^{2}}={{A}^{2}}{{(x-\bar{x})}^{2}}\]             Þ \[\sum {{(y-\bar{y})}^{2}}={{A}^{2}}\sum {{(x-\bar{x})}^{2}}\]             Þ \[n.\sigma _{y}^{2}={{A}^{2}}.n\sigma _{x}^{2}\] Þ \[\sigma _{y}^{2}={{A}^{2}}\sigma _{x}^{2}\]             Þ \[{{\sigma }_{y}}=\,|A|{{\sigma }_{x}}\] Þ \[{{\sigma }_{y}}=\,\left| \frac{a}{c} \right|{{\sigma }_{x}}\]             Thus, new S.D.\[=\left| \frac{a}{c} \right|\,\sigma \].


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