KVPY Sample Paper KVPY Stream-SX Model Paper-5

  • question_answer
                                                If \[0<\theta ,\phi <\pi /2\] and \[x=\sum\limits_{n=0}^{\infty }{{{\sin }^{2n}}\theta },y=\sum\limits_{n=0}^{\infty }{{{\cos }^{2n}}\phi }\] and \[z=\sum\limits_{n=0}^{\infty }{{{\cos }^{n}}\left( \theta +\phi  \right){{\cos }^{n}}\left( \theta +\phi  \right)},\] then

    A) \[xyz+1=yz-zx\]

    B) \[xyz-1=yz+zx\]

    C) \[xyz-xy=yz-zx\]

    D) \[xyz+1=yz+zx\]

    Correct Answer: C

    Solution :

    [c]
    Given, \[x=\sum\limits_{n=0}^{\infty }{{{\sin }^{2n}}\theta }\] \[=1+{{\sin }^{2}}\theta +{{\sin }^{4}}\theta +...\]
    \[\Rightarrow \]\[x=\frac{1}{1-{{\sin }^{2}}\theta }=\frac{1}{{{\cos }^{2}}\theta }\]
    \[\Rightarrow \]\[y=\sum\limits_{n=0}^{\infty }{{{\cos }^{2n}}\phi }\]\[=1+{{\cos }^{2}}\phi +{{\cos }^{4}}\phi +...\]
    \[\Rightarrow y=\frac{1}{1-{{\cos }^{2}}\phi }=\frac{1}{{{\sin }^{2}}\phi }\]
    \[z=\sum\limits_{n=0}^{\infty }{{{\cos }^{n}}\left( \theta +\phi  \right)}{{\cos }^{n}}\left( \theta +\phi  \right)\]
    \[\Rightarrow \]\[z=1+\cos (\theta +\phi ){{\cos }^{2}}(\alpha -\phi )\]\[+{{\cos }^{2}}(\theta +\phi ){{\cos }^{2}}(\alpha -\phi )+...\]
    \[\Rightarrow \]\[z=\frac{1}{1-\cos (\theta +\phi )\cos (\theta -\phi )}\]
    \[\Rightarrow \]\[z=\frac{1}{1-\left( \frac{1}{x}-\frac{1}{y} \right)}=\frac{xy}{xy-(y-x)}\]
    \[\Rightarrow \]\[xyz-(zy-zx)=xy\]\[\Rightarrow \]\[xyz-xy=yz-xz\]


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