CEE Kerala Engineering CEE Kerala Engineering Solved Paper-2000

  • question_answer
    If\[\alpha ,\beta \]are the roots of\[a{{x}^{2}}+bx+c=0\]and \[\alpha +k,\text{ }\beta +k\]are the roots of\[p{{x}^{2}}+qx+r=0,\] then\[\frac{{{b}^{2}}-4ac}{{{q}^{2}}-4pr}\]is equal to:

    A)  \[\frac{a}{p}\]                 

    B)         \[1\]                    

    C)  \[{{\left( \frac{a}{p} \right)}^{2}}\]

    D)         \[0\]

    E)         \[{{\left( \frac{p}{a} \right)}^{2}}\]

    Correct Answer: C

    Solution :

                    Since,\[\alpha ,\beta \]are the roots of the equation \[ax+bx+c=0\] Then,    \[\alpha =\frac{-b}{2a}+\frac{\sqrt{{{b}^{2}}-4ac}}{2a},\]                 \[\beta =\frac{-b}{2a}-\frac{\sqrt{{{b}^{2}}-4ac}}{2a},\]                 ??(i) And\[\alpha +k,\beta +k\]are the roots of the equation \[p{{x}^{2}}+qx+r=0\] then, \[\alpha +k=-\frac{q}{2p}+\frac{\sqrt{{{q}^{2}}-4pr}}{2p}\] and \[\beta +k=-\frac{q}{2p}-\frac{\sqrt{{{q}^{2}}-4pr}}{2p}\] \[\Rightarrow \]\[k=-\frac{q}{2p}+\frac{\sqrt{{{q}^{2}}-4pr}}{2p}+\frac{b}{2a}-\frac{\sqrt{{{b}^{2}}-4ac}}{2a}\] [ from (i)] and \[k=-\frac{q}{2p}-\frac{\sqrt{{{q}^{2}}-4pr}}{2p}+\frac{b}{2a}-\frac{\sqrt{{{b}^{2}}-4ac}}{2a}\] [from (i)] \[\Rightarrow \]               \[\frac{\sqrt{{{q}^{2}}-4pr}}{2p}-\frac{\sqrt{{{b}^{2}}-4ac}}{2a}\]                 \[=\frac{-\sqrt{{{q}^{2}}-4pr}}{2p}+\frac{\sqrt{{{b}^{2}}-4ac}}{2a}\] \[\Rightarrow \]               \[\frac{\sqrt{{{q}^{2}}-4pr}}{p}=\frac{\sqrt{{{b}^{2}}-4ac}}{a}\]  \[\Rightarrow \]               \[\frac{{{q}^{2}}-4pr}{{{p}^{2}}}=\frac{{{b}^{2}}-4ac}{{{a}^{2}}}\] \[\therefore \]  \[\frac{{{b}^{2}}-4ac}{{{q}^{2}}-4pr}={{\left( \frac{a}{p} \right)}^{2}}\]


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