JEE Main & Advanced Mathematics Functions Question Bank Self Evaluation Test - Relation and Functions-II

  • question_answer
    If \[f(x)=x-{{x}^{2}}+{{x}^{3}}-{{x}^{4}}+...to\] \[\infty \] for \[\left| x \right|<1,\] then \[{{f}^{-1}}(x)=\]

    A) \[\frac{x}{1+x}\]

    B) \[\frac{x}{1-x}\]

    C) \[\frac{1-x}{x}\]

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

    Correct Answer: B

    Solution :

    [b] Given \[f(x)=x-{{x}^{2}}+{{x}^{3}}-{{x}^{4}}+...to\infty \] \[\Rightarrow y=\frac{x}{1+x}\] (Infinite G.P.) \[\Rightarrow y+xy=x\Rightarrow y=x(1-y)\] \[\Rightarrow x=\frac{y}{1-y}\] \[\Rightarrow {{f}^{-1}}(y)=\frac{y}{1-y}\Rightarrow {{f}^{1}}(x)=\frac{x}{1-x}\]


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