JEE Main & Advanced Mathematics Applications of Derivatives Question Bank Mock Test - Application of Derivatives

  • question_answer
    If the function \[f(x)=a{{x}^{3}}+b{{x}^{2}}+11x-6\] satisfies condition of Rolle's theorem in [1, 3] for \[x=2+\frac{1}{\sqrt{3}}\], then values of a and b , respectively, are

    A) \[-\,3,\text{ }2\] 

    B) \[2,\,\,-4\]

    C) 1, 6                  

    D) none of these

    Correct Answer: D

    Solution :

    [d] \[f(x)=a{{x}^{2}}+b{{x}^{2}}+11x-6\] Satisfies condition of Roll's theorem in [1, 3]. Therefore, \[f(1)=f(3)\] or  \[a+b+11-6=27a+9b+33-6\] or \[13a+4b=-11\]                                  ...(1) And \[f'(x)=3a{{x}^{2}}+2bx+11\] or \[f'\left( 2+\frac{1}{\sqrt{3}} \right)=3a{{\left( 2+\frac{1}{\sqrt{3}} \right)}^{2}}+2b\left( 2+\frac{1}{\sqrt{3}} \right)+11=0\]or \[3a\left( 4+\frac{1}{3}+\frac{1}{\sqrt{3}} \right)+2b\left( 2+\frac{1}{\sqrt{3}} \right)+11=0\]                                                      ...(2) From equations (1) and (2), We get \[a=1,\text{ }b=-\,6.\]


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