JEE Main & Advanced Mathematics Sequence & Series Question Bank Self Evaluation Test - Sequences and Series

  • question_answer
    If \[\left| \,r\, \right|>1\] and \[x=a+\frac{a}{r}+\frac{a}{{{r}^{2}}}+....to\,\,\infty \],\[y=b-\frac{b}{r}+\frac{b}{{{r}^{2}}}-....\,to\,\,\infty \]and \[z=c+\frac{c}{{{r}^{2}}}+\frac{c}{{{r}^{4}}}+....to\,\,\infty \]then \[\frac{xy}{z}=\]

    A) \[\frac{ab}{c}\]

    B) \[\frac{ac}{b}\]

    C) \[\frac{bc}{a}\]

    D) 1

    Correct Answer: A

    Solution :

    [a] Since \[\left| r \right|>1,\frac{1}{\left| r \right|}<1\] \[\therefore \,\,\,\,\,x=\frac{a}{1-\frac{1}{r}}=\frac{ar}{r-1}\] Similarly, \[y=\frac{b}{1-\left( -\frac{1}{r} \right)}=\frac{br}{r+1}\] and \[z=\frac{c}{1-\frac{1}{{{r}^{2}}}}=\frac{c{{r}^{2}}}{{{r}^{2}}-1}\]                                  ?. (1) \[\therefore xy=\frac{ar}{r-1}\times \frac{br}{r+1}=\frac{ab{{r}^{2}}}{{{r}^{2}}-1}\]  ?. (2)             Dividing (2) by (1), we get                         \[\frac{xy}{z}=\frac{ab{{r}^{2}}}{{{r}^{2}}-1}\times \frac{{{r}^{2}}-1}{c{{r}^{2}}}=\frac{ab}{c}\]


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