JEE Main & Advanced Sample Paper JEE Main - Mock Test - 40

  • question_answer
    \[\sin x+\cos x={{y}^{2}}-y+a\] has no value of x for any a if a belongs to

    A) \[(0,\sqrt{3})\]

    B)        \[(\sqrt{3},\infty )\]

    C) \[(-\infty ,\,-\sqrt{3})\]

    D)        \[(-\sqrt{3},0)\]

    Correct Answer: B

    Solution :

    [b] \[{{y}^{2}}-y+a={{\left( y-\frac{1}{2} \right)}^{2}}+a-\frac{1}{4}\] Since \[-\sqrt{2}\le \sin x+\cos x\le \sqrt{2},\] given equation will have no real value of x for any y if \[a-\frac{1}{4}>\sqrt{2}.\] i.e.,       \[a\in \left( \sqrt{2}+\frac{1}{4},\infty  \right)\] \[\Rightarrow \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,a\in (\sqrt{3},\infty )\]   \[(as\,\,\sqrt{2}+1/4<\sqrt{3})\]


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