JEE Main & Advanced Mathematics Functions Question Bank Functions

  • question_answer
    If \[g(x)={{x}^{2}}+x-2\] and \[\frac{1}{2}gof(x)=2{{x}^{2}}-5x+2\], then \[f(x)\] is [Roorkee 1998; MP PET 2002]

    A)                    \[2x-3\]

    B)            \[2x+3\]

    C)                    \[2{{x}^{2}}+3x+1\]

    D)            \[2{{x}^{2}}-3x-1\]

    Correct Answer: A

    Solution :

               \[\frac{1}{2}\,(gof)\,(x)=2{{x}^{2}}-5x+2\]or \[\frac{1}{2}\,g\,[f\,(x)]=2{{x}^{2}}-5x+2\]            \[\underset{x\to 0}{\mathop{\lim }}\,\,\frac{(1+nx+{{\,}^{n}}{{C}_{2}}{{x}^{2}}+.....\text{higher powers of }x\text{ to }{{x}^{n}})-1}{x}\]            \[\Rightarrow \,\,f{{(x)}^{2}}+f(x)-(4{{x}^{2}}-10x+6)=0\]            \[\therefore \]\[f(x)=\frac{-1\pm \sqrt{1+4\,(4{{x}^{2}}-10x+6)}}{2}\]                      \[=\frac{-1\pm \sqrt{(16{{x}^{2}}-40x+25}}{2}=\frac{-1+(4x-5)}{2}=2x-3\].


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