BCECE Engineering BCECE Engineering Solved Paper-2015

  • question_answer
    If \[\frac{\log 3}{x-y}=\frac{\log 5}{y-z}=\frac{1og7}{z-x}\],then \[{{3}^{x+y}}{{.5}^{y+z}}{{.7}^{z+x}}\] is equal to

    A)  0                                            

    B) 2

    C) 1                                             

    D) None of these

    Correct Answer: C

    Solution :

     We have, \[\frac{\log 3}{x-y}=\frac{\log 5}{y-z}=\frac{\log 7}{z-x}=\lambda \][say] \[\Rightarrow \]\[\log 3=\lambda (x-y),\log 5=\lambda (y-z),\log 7\] \[=\lambda (z-x)\]                 \[\Rightarrow \]\[3={{10}^{\lambda (x-y)}},5={{10}^{\lambda (y-z)}},7={{10}^{\lambda (z-x)}}\]                 \[\Rightarrow \]\[{{3}^{x+y}}{{.5}^{y+z}}{{.7}^{z+x}}={{10}^{\lambda ({{x}^{2}}-{{y}^{2}})}}\]                                                 \[{{.10}^{\lambda ({{y}^{2}}-{{z}^{2}})}}{{.10}^{\lambda ({{z}^{2}}-{{x}^{2}})}}\]                 \[\Rightarrow \]\[{{3}^{x+y}}{{.5}^{y+z}}{{.7}^{z+x}}\]                                 \[={{10}^{\lambda ({{x}^{2}}-{{y}^{2}}+{{y}^{2}}-{{z}^{2}}+{{z}^{2}}-{{x}^{2}})}}\]                 \[\Rightarrow \]\[{{3}^{x+y}}{{.5}^{y+z}}{{.7}^{z+x}}={{10}^{0}}\]                 \[\Rightarrow \]\[{{3}^{x+y}}{{.5}^{y+z}}{{.7}^{z+x}}=1\]


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