JEE Main & Advanced Mathematics Functions Question Bank Differentiability

  • question_answer
    Let                                                                                                     \[f\]  be differentiable for all                                                                                                     \[x\] . If                                                                                                \[f(1)=-2\]  and                                                                                              \[f'(x)\ge 2\]  for                                                                                               \[x\in [1,6]\] , then [AIEEE 2005]

    A)                                                                                                 \[f(6)<5\]

    B)                                                                                                 \[f(6)=5\]

    C)                                                                                               \[f(6)\ge 8\]

    D)                                                                                                 \[f(6)<8\]

    Correct Answer: C

    Solution :

               As                                                                                                \[f(1)=-2\]  and                                                                                 \[f'(x)\ge 2\forall x\in [1,6]\]            Applying lagrange?s mean value theorem,                                                                                                   \[\frac{f(6)-f(1)}{5}\]                                                                                             \[=f'(c)\ge 2\]                                        \[\Rightarrow f(6)\ge 10+f(1)\Rightarrow f(6)\ge 10-2\Rightarrow f(6)\ge 8\].


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