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

  • question_answer
    If \[x{{.}^{\ell n\left( \frac{y}{z} \right)}}.{{y}^{_{^{\ell n{{(XZ)}^{2}}}}}}.\,{{z}^{\ell n\left( \frac{x}{y} \right)}}={{y}^{4\,\ell n\,y}}\] for any x > 1, y >1 and z > 1, then which one of the following is correct?

    A) \[\ell n\,y\] is the GM of \[\ell n\,x,\,\ell n\,x,\,\ell n\,x\] and \[\ell n\,z\]

    B) \[\ell n\,y\] is the AM of \[\ell n\,x,\,\ell n\,x,\,\ell n\,x\]and \[\ell n\,z\]

    C) \[\ell n\,y\] is the HM of \[\ell n\,x,\,\ell n\,x,\,\ell n\,x\] and \[\ell n\,z\]

    D) \[\ell n\,y\] is the AM of \[\ell n,\,\,In\,\,x,\,\,\ell n\,z\] and \[\ell n\,z\]

    Correct Answer: B

    Solution :

    [d] \[{{x}^{ln\left( \frac{y}{z} \right)}}.\,\,{{y}^{ln{{(xz)}^{2}}}}.\,\,{{z}^{ln\left( \frac{x}{y} \right)}}={{y}^{4\,\,ln\,\,y}}\] \[\Rightarrow \,\,ln\left[ {{z}^{ln\left( \frac{y}{z} \right)}} \right]+ln\,\left[ {{y}^{ln{{(xz)}^{2}}}} \right]+ln\left[ {{z}^{ln\left( \frac{x}{y} \right)}} \right]=ln\,\,\left[ {{y}^{4\,\,ln\,\,y}} \right]\] \[\Rightarrow \,\,\left[ ln\left( \frac{y}{z} \right)ln\,x \right]+[2\,\,ln\,\,(xz)ln\,\,y]+\left[ ln\left( \frac{x}{y} \right)ln\,\,z \right]=4\,{{[ln\,\,y]}^{2}}\]\[\Rightarrow \,\,ln\,\,x[ln\,\,y-ln\,\,z]+2\,\,ln\,\,y[ln\,\,x+\,\,ln\,\,z]\] \[+\,ln\,\,z[ln\,\,x-ln\,\,y]=4\,\,{{[ln\,\,y]}^{2}}\] \[\Rightarrow \,\,\,\,3\,\,ln\,\,x+ln\,\,z=4\,\,ln\,\,y\] \[\Rightarrow \frac{ln\,\,x+ln\,\,x=ln\,\,x+ln\,\,z}{4}\,\,=\,\,ln\,\,y\] \[\therefore \]    lny is the AM of lnx, lnx, lnx & lnz.


You need to login to perform this action.
You will be redirected in 3 sec spinner