JEE Main & Advanced AIEEE Solved Paper-2004

  • question_answer
    If\[2a+3b+6c=0,\] then atleast one root of the equation\[a{{x}^{2}}+bx+c=0\]lies in the interval

    A) (0, 1)                     

    B) (1, 2)     

    C)        (2, 3)     

    D)        (1, 3)

    Correct Answer: A

    Solution :

    Let \[f'(x)=a{{x}^{2}}+bx+c\] \[\Rightarrow \]\[f(x)=\frac{a{{x}^{3}}}{3}+\frac{b{{x}^{2}}}{2}+cx+d\] \[\Rightarrow \]\[f(x)=\frac{2a{{x}^{3}}+3b{{x}^{2}}+6cx+6d}{6}\] Since,\[f(x)\]is a polynomial function and is continuous as well as differentiable in its entire real set. \[\Rightarrow \]\[f(1)=\frac{2a+3b+6c+6d}{6}=\frac{6d}{6}=d\]                                                 \[(\because 2a+3b+6c=0)\] and        \[f(0)=\frac{6d}{6}=d\] \[\therefore \]  \[f(0)=f(1)\] \[\Rightarrow \]\[f'(x)\]will vanish at least once between 0 and 1. \[\therefore \]One of the roots of\[a{{x}^{2}}+bx+c=0\]lies between 0 and 1.


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