KVPY Sample Paper KVPY Stream-SX Model Paper-2

  • question_answer
    If x, y, z are unequal positive numbers in H.P., then \[{{x}^{3}}+{{z}^{3}}\]

    A) \[\ge 2{{y}^{3}}\]                   

    B) \[\le 2{{y}^{3}}\]

    C) \[>2{{y}^{3}}\]         

    D) none of these

    Correct Answer: C

    Solution :

    \[\Rightarrow \]   x, y, z are in H.P
    \[\Rightarrow \]   y = H.M. of x and z
    \[\because \]       G.M. < H.M.
    \[\Rightarrow \]   \[\sqrt{xz}>y\]
    Again,   Since A.M. > G.M.
    (here x, y, z are unequal)
    \[\therefore \]      \[\frac{{{x}^{3}}+{{z}^{3}}}{2}>\sqrt{{{(xz)}^{3}}}>{{y}^{3}}\]
    \[\Rightarrow \]   \[{{x}^{3}}+{{z}^{3}}>2{{y}^{3}}\]         


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