JAMIA MILLIA ISLAMIA Jamia Millia Islamia Solved Paper-2015

  • question_answer
    If 3 arithmetic means, 3 geometric means and 3 harmonic means are inserted between and 5, then the cubic equation whose roots are first AM, second GM and third HM between 1 and 5, is

    A) \[f\]

    B) \[{{f}^{-1}}\]

    C) \[f(x)=\left\{ \begin{matrix}    2x & x<0  \\    2x+1 & x\ge 0  \\ \end{matrix}, \right.\]

    D) None of the above

    Correct Answer: A

    Solution :

    Let a = first AM between 1 and 5 \[{{x}^{2}}+{{y}^{2}}-8x-6y=0\]  \[2{{x}^{2}}+2{{y}^{2}}-x-7y=0\] b = second GM between 1 and 5 \[{{x}^{2}}+{{y}^{2}}-6x-10y=0\]               \[{{\left( px+\frac{1}{x} \right)}^{n}}\] and c = third HM between 1 and 5 \[n\in N\]            \[\frac{5}{2}\] Now, \[a+b+c=2+\sqrt{5}+\frac{5}{2}\] \[ab+bc+ca=2\sqrt{5}+\frac{5\sqrt{5}}{2}+5\] And        \[abc=5\sqrt{5}\] The cubic equation having a, b, c as its roots, is \[{{x}^{3}}-(a+b+c){{x}^{2}}+(ab+bc+ca)x-abc=0\] \[\Rightarrow \]\[{{x}^{3}}-\left( \frac{9}{2}+\sqrt{5} \right){{x}^{2}}+\left( \frac{9\sqrt{5}}{2}+5 \right)x-5\sqrt{5}=0\]


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