JEE Main & Advanced Mathematics Straight Line Question Bank Self Evaluation Test - Straight Lines & Pair Striaght Lines

  • question_answer
    Let \[A\left( \alpha ,\frac{1}{\alpha } \right),B\left( \alpha ,\frac{1}{\beta } \right),C\left( \gamma ,\frac{1}{\gamma } \right)\] be the vertices of a \[\Delta ABC,\] where \[\alpha ,\beta \] are the roots of the equation \[{{x}^{2}}-6{{p}_{1}}x+2=0,\,\,\beta ,\,\,\gamma \]\[{{x}^{2}}-6{{p}_{1}}x+2=0,\beta ,\gamma \] are the roots of the equation \[{{x}^{2}}-6{{p}_{2}}x+3=0\] and \[\gamma ,\alpha \] are the roots of the equation \[{{x}^{2}}-6{{p}_{3}}x+6=0,{{p}_{1}},{{p}_{2}},{{p}_{3}}\] being positive. Then, the coordinates of the centroid of \[\Delta ABC\] is

    A) \[\left( 1,\frac{11}{18} \right)\]

    B) \[\left( 0,\frac{11}{8} \right)\]

    C) \[\left( 2,\frac{11}{18} \right)\]

    D) None of these

    Correct Answer: C

    Solution :

    [c] it is given that \[\alpha ,\beta \] are the roots of the equation \[{{x}^{2}}-6{{p}_{1}}x+2=0.\]
    \[\therefore \alpha +\beta =6{{p}_{1}},\alpha \beta =2\]                          ? (i)
    \[\beta ,\gamma \] are the roots of the equation \[{{x}^{2}}-6{{p}_{2}}x+3=0.\]
    \[\therefore \beta +\gamma =6{{p}_{2}},\beta \gamma =3\]                                 ?. (ii)
    \[\gamma ,\alpha \] are the roots of the equation \[{{x}^{2}}-6{{p}_{3}}x+6=0.\]
    \[\therefore \,\,\,\,\gamma +\alpha =6{{p}_{3}},\,\,\,\gamma \alpha =6\]                           ?. (iii)
    From Eqs. (i), (ii) and (iii), we get
    \[\Rightarrow \alpha \beta \gamma =6\]                 \[[\therefore \alpha ,\beta ,\gamma >0]\]
    Now, \[\alpha \beta =2\] and \[\alpha \beta \gamma =6\]
    \[\Rightarrow \gamma =3\]
    \[\beta \gamma =3\] and \[\alpha \beta \gamma =6\]
    \[\alpha =3\]      \[\alpha =6\alpha \beta \gamma =6\]
    \[\Rightarrow \beta =1\]
    \[\therefore \,\,\,\,\,\,\alpha +\beta =6{{p}_{1}}\Rightarrow 3=6{{p}_{1}}\]
    \[\Rightarrow {{p}_{1}}=\frac{1}{2}\]
    \[\beta +\gamma =6{{p}_{2}}\Rightarrow 4=6{{p}_{2}}\]
    \[\Rightarrow \,\,\,\,{{p}_{2}}=\frac{2}{3}\]
    and \[\gamma +\alpha =6{{p}_{3}}\Rightarrow 5=6{{p}_{3}}\]
    \[\Rightarrow {{p}_{3}}=\frac{5}{6}\]
    The coordinates of the centroid of triangle are
    \[\left( \frac{\alpha +\beta +\gamma }{3},\frac{1}{3}\left( \frac{1}{\alpha }+\frac{1}{\beta }+\frac{1}{\gamma } \right) \right)\] or
    \[\left( \frac{6}{3},\frac{1}{3}\left( \frac{1}{2}+1+\frac{1}{3} \right) \right)or\left( 2,\frac{11}{18} \right)\]


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