JEE Main & Advanced Mathematics Relations Question Bank Mock Test - Relations and Functions

  • question_answer
    If \[f(x)=\left\{ \begin{matrix}    {{x}^{2}},\,\,\,for\,x\ge 0  \\    x,\,\,\,\,for\,x<0  \\ \end{matrix} \right.,\] then fof (x) is given by

    A) \[{{x}^{2}}for\,x\ge 0,\,\]\[x\text{ }for\text{ }x<0\]

    B) \[{{x}^{4}}for\,x\ge 0,\,\]\[{{x}^{2}}for\text{ }x<0\]

    C) \[{{x}^{4}}for\,x\ge 0,\,\]\[-{{x}^{2}}for\text{ }x<0\]

    D) \[{{x}^{4}}for\,x\ge 0,\,\]\[x\text{ }for\text{ }x<0\]

    Correct Answer: D

    Solution :

    [d] \[f(f(x))=\left\{ \begin{matrix}    {{(f(x))}^{2}},\text{for}\,f(x)\ge 0  \\    f(x),\,\,\,\text{for}\,\,f(x)<0  \\ \end{matrix} \right.\] =\[\left\{ \begin{matrix}    {{({{x}^{2}})}^{2}},{{x}^{2}}\ge 0,x\ge 0  \\    {{x}^{2}},\,\,x\ge 0,\,\,x<0  \\    {{x}^{2}},\,\,{{x}^{2}}<0,\,\,x\ge 0  \\    x,\,\,x<0,\,\,x<0  \\ \end{matrix} \right.\] \[=\left\{ \begin{matrix}    {{x}^{4}},\,\,x\ge 0  \\    x,\,\,x<0  \\ \end{matrix} \right.\]


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