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JEE Main & Advanced Sample Paper JEE Main - Mock Test - 8

  • question_answer
    If for the curve \[y=1+bx-{{x}^{2}}\], the tangent at (1, -2) is parallel to x-axis, then b =

    A)  2                    

    B) -2   

    C) 1                     

    D) -1

    Correct Answer: A

    Solution :

    [a] : Given, equation of curve is \[y=1+bx-{{x}^{2}}\]\[\Rightarrow \]\[\frac{dy}{dx}=b-2x.\]\[\Rightarrow \]\[{{\left. \frac{dy}{dx} \right|}_{(1,-2)}}=b-2\] Since, tangent at (1, -2) is parallel to x-axis \[\therefore \]\[{{\left. \frac{dy}{dx} \right|}_{(1,-2)}}=0\Rightarrow b-2=0\Rightarrow b=2\].


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