BCECE Engineering BCECE Engineering Solved Paper-2006

  • question_answer
    If \[\alpha ,\beta \]are the roots of \[a{{x}^{2}}+bx+c=0,\alpha +h,\beta +h\] are roots of \[p{{x}^{2}}+qx+r=0;\]and\[{{D}_{1}},{{D}_{2}}\]are the respective discriminants of the equations, then \[{{D}_{1}}:{{D}_{2}}\]is equal to:

    A) \[\frac{{{a}^{2}}}{{{p}^{2}}}\]                    

    B)         \[\frac{{{b}^{2}}}{{{q}^{2}}}\]                   

    C)  \[\frac{{{c}^{2}}}{{{r}^{2}}}\]                     

    D)         none of these

    Correct Answer: A

    Solution :

    Given\[\alpha ,\beta \] be the roots of \[a{{x}^{2}}+bx+c=0\]and\[\alpha +h,\beta +h\] are the roots of \[p{{x}^{2}}+qx+r=0\] and also\[{{D}_{1}},{{D}_{2}}\] discriminants of respectively equations. Let    \[A=\alpha +h\]and \[B=\beta +h\] Then,    \[A-B=(\alpha +h)-(\beta +h)=\alpha -\beta \] \[\Rightarrow \]               \[{{(A-B)}^{2}}={{(\alpha -\beta )}^{2}}\] \[\Rightarrow \]               \[{{(A+B)}^{2}}-4AB={{(\alpha +\beta )}^{2}}-4\,\alpha \beta \] \[\Rightarrow \]               \[\frac{{{q}^{2}}}{{{p}^{2}}}-\frac{4r}{p}=\frac{{{b}^{2}}}{{{a}^{2}}}-\frac{4c}{a}\] \[\Rightarrow \]               \[\frac{{{q}^{2}}-4rp}{{{p}^{2}}}=\frac{{{b}^{2}}-4ac}{{{a}^{2}}}\] \[\Rightarrow \]               \[\frac{{{D}_{2}}}{{{p}^{2}}}=\frac{{{D}_{1}}}{{{a}^{2}}}\] \[(\because \,{{D}_{1}}={{b}^{2}}-4ac,{{D}_{2}}={{q}^{2}}-4rp)\] \[\Rightarrow \]               \[\frac{{{D}_{1}}}{{{D}_{2}}}=\frac{{{a}^{2}}}{{{p}^{2}}}\]


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