JEE Main & Advanced JEE Main Solved Paper-2014

  • question_answer
    Let \[{{f}_{k}}(x)=\frac{1}{k}(si{{n}^{k}}x+co{{s}^{k}}x),\]where\[x\in R\]and\[k\ge 1.\]Then\[{{f}_{4}}(x)-{{f}_{6}}(x)\]equals:   JEE Main  Solved  Paper-2014

    A) \[\frac{1}{6}\]                                   

    B) \[\frac{1}{3}\]

    C) \[\frac{1}{4}\]                                   

    D) \[\frac{1}{12}\]

    Correct Answer: D

    Solution :

    \[{{f}_{k}}=\frac{1}{4}\left( {{\sin }^{k}}x+{{\cos }^{k}}x \right)\] \[{{f}_{6}}(x)=\frac{1}{6}\left( {{\sin }^{6}}x+{{\cos }^{6}}x \right)\]
    \[{{f}_{4}}(x)=\frac{1}{4}\left( {{\sin }^{6}}x+{{\cos }^{4}}x \right)\] \[{{f}_{6}}K=\frac{1}{6}\left[ 1-\frac{3}{4}{{\sin }^{2}}2x \right]\]
    \[{{f}_{4}}(x)=\frac{1}{4}\left[ 1-\frac{{{\sin }^{2}}2x}{2} \right]\]  
    \[{{f}_{4}}(x)-{{f}_{6}}(x)=\left[ \frac{1}{4}-\frac{{{\sin }^{2}}2x}{8} \right]-\left[ \frac{1}{6}-\frac{{{\sin }^{2}}2x}{8} \right]=\frac{1}{4}-\frac{1}{6}=\frac{1}{12}\]


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