JAMIA MILLIA ISLAMIA Jamia Millia Islamia Solved Paper-2015

  • question_answer
    The locus of the points of intersection of the tangents at the extremities of the chords of the ellipse\[l=\frac{L}{200}\times stored\,\,energy\]which touches the ellipse \[Y=\frac{1}{2}\times 3\times {{10}^{10}}\times {{\left( \frac{L}{200L} \right)}^{2}}\], is

    A) \[Y=\frac{1}{2}\times 3\times {{10}^{10}}\times \frac{1}{4\times {{10}^{4}}}=\frac{3}{8}\times {{10}^{6}}\]

    B) \[=0.375\times {{10}^{6}}\]

    C) \[=3.75\times {{10}^{5}}\]

    D) None of the above

    Correct Answer: C

    Solution :

    Given equation of second ellipse can be rewritten as \[\frac{{{x}^{2}}}{4}+\frac{{{y}^{2}}}{1}=1\] Equation of tangent to the ellipse is \[\frac{x}{2}\cos \theta +y\sin \theta =1\]            ...(i) Equation of the first ellipse can be rewritten as \[\frac{{{x}^{2}}}{6}+\frac{{{y}^{2}}}{3}=1\]             ??.. (ii) Let ellipse (i) meets the first ellipse at P and Q and the tangents at P and Q to the second ellipse intersected at \[(h,k)\], then Eq, (i) is the chord of contact of (A, k) with respect to the ellipse (ii). Thus. its equation is \[\frac{hx}{6}+\frac{ky}{3}=1\]                  ?? (ii) Since, Eqs. (i) and (iii) represent the same line. \[\frac{h/6}{\cos \frac{\theta }{2}}=\frac{k/3}{\sin \theta }=1\] Now,     \[{{\text{h}}^{\text{2}}}+{{\text{k}}^{\text{2}}}={{\text{3}}^{\text{2}}}\text{co}{{\text{s}}^{\text{2}}}\theta +\text{ }{{\text{3}}^{\text{2}}}\text{si}{{\text{n}}^{\text{2}}}\theta \] \[=\text{9}\left( \text{1} \right)=\text{9}\] Hence, locus is \[{{x}^{2}}+{{y}^{2}}=9\].


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