JEE Main & Advanced JEE Main Online Paper (Held on 9 April 2013)

  • question_answer
                                    Let\[f(x)=\frac{{{x}^{2}}-x}{{{x}^{2}}+2x},x\ne 0,-2.\]Then\[\frac{d}{dx}\left[ {{f}^{-1}}(x) \right]\] (wherever it is defined) is equal to :        JEE Main Online Paper (Held On 09 April 2013)                         

    A)                 frac{-1}{{{(1-x)}^{2}}}\]                

    B)                 \[\frac{3}{{{(1-x)}^{2}}}\]                

    C)                 \[\frac{1}{{{(1-x)}^{2}}}\]                

    D)                 \[\frac{-3}{{{(1-x)}^{2}}}\]                                           

    Correct Answer: B

    Solution :

                    Let \[y=\frac{{{x}^{2}}-x}{{{x}^{2}}+2x}\] \[\Rightarrow \]               \[x=\frac{2y+1}{-y+1};\,x\ne 0\] \[\Rightarrow \]               \[{{f}^{-1}}(x)=\frac{2x+1}{-x+1}\] \[\therefore \]  \[\frac{d}{dx}\{{{f}^{-1}}(x)=\frac{(-x+1)\cdot 2-(2x+1)(-1)}{{{(-x+1)}^{2}}}\]                 \[=\frac{3}{{{(-x+1)}^{2}}}\]                


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