JEE Main & Advanced Mathematics Functions Question Bank Functions

  • question_answer
    If \[f(x)=3x-5\], then \[{{f}^{-1}}(x)\] [IIT 1998]

    A)                    Is given by \[\frac{1}{3x-5}\]                                     

    B)                    Is given by \[\frac{x+5}{3}\]

    C)                    Does not exist because f  is not one-one

    D)                    Does not exist because f is not onto

    Correct Answer: B

    Solution :

               Let \[f(x)=y\,\,\Rightarrow \,\,x={{f}^{-1}}(y).\]            Hence\[f(x)=y=3x-5\,\,\Rightarrow \,\,x=\frac{y+5}{3}\,\Rightarrow \,{{f}^{-1}}(y)=x=\frac{y+5}{3}\]            \[\therefore \,\,{{f}^{-1}}(x)=\frac{x+5}{3}\]                    Also f is one-one and onto, so \[{{f}^{-1}}\] exists and is given by \[{{f}^{-1}}(x)=\frac{x+5}{3}\].


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