JEE Main & Advanced Mathematics Complex Numbers and Quadratic Equations Question Bank Condition for common roots, Quadratic expressions and Position of roots

  • question_answer
    If \[x\] be real, then the maximum value of \[5+4x-4{{x}^{2}}\] will be equal to [MNR 1979]

    A) 5

    B) 6

    C) 1

    D)   2

    Correct Answer: B

    Solution :

     Let  \[f(x)=5+4x-4{{x}^{2}}=y\,\,\,\,\Rightarrow \,\,4{{x}^{2}}-4x-5+y=0\] Since x is real, so \[{{B}^{2}}-4AC\ge 0\] Þ \[16-4.4(-5+y)\ge 0\] Þ \[16-16(-5+y)\ge 0\,\,\Rightarrow \,\,-5+y\le 1\,\,\Rightarrow y\le 6\] Hence maximum value of y is 6.


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