JEE Main & Advanced Mathematics Sequence & Series Question Bank Arithmetic Progression

  • question_answer
    If \[x,y,z\] are in A.P. and \[{{\tan }^{-1}}x,{{\tan }^{-1}}y\]and \[{{\tan }^{-1}}z\] are also in A.P., then [Kerala (Engg.) 2005]

    A) \[x=y=z\]

    B) \[x=y=-z\]

    C) \[x=1;y=2;z=3\]

    D) \[x=2;y=4;z=6\]

    E) \[x=2y=3z\]

    Correct Answer: A

    Solution :

    \[2{{\tan }^{-1}}y={{\tan }^{-1}}x+{{\tan }^{-1}}\]z Þ\[{{\tan }^{-1}}\left( \frac{2y}{1-{{y}^{2}}} \right)={{\tan }^{-1}}\left( \frac{x+z}{1-xz} \right)\] Þ\[\frac{2y}{1-{{y}^{2}}}=\frac{x+z}{1-xz}\] But \[2y=x+z\] \\[1-{{y}^{2}}=1-xz\]Þ \[{{y}^{2}}=xz\] \[\because xyz\] are both in G.P. and A.P.,  \ \[x=y=z\].


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