JCECE Engineering JCECE Engineering Solved Paper-2013

  • question_answer
    If the slope of one of the lines represented by the equation \[a{{x}^{2}}+2hxy+b{{y}^{2}}=0\] be square of the other, then the value of\[\frac{a+b}{h}+\frac{8{{h}^{2}}}{ab}\]is

    A) \[4\]                                     

    B) \[-6\]

    C) \[6\]                                     

    D) \[-4\]

    Correct Answer: B

    Solution :

    Let the equation of the lines by \[y={{m}_{1}}x\] and \[y={{m}_{2}}x\], then                       \[{{m}_{2}}=m_{1}^{2}\]                ... (i) Also,      \[{{m}_{1}}+{{m}_{2}}=\frac{2h}{b}\]                 \[{{m}_{1}}+m_{1}^{2}=\frac{-2h}{b}\]                                  ... (ii) and        \[{{m}_{1}}{{m}_{2}}=\frac{a}{b}\Rightarrow m_{1}^{3}=\frac{a}{b}\]                     ... (iii) Taking cube of both the sides of the Eq. (ii), we get                 \[m_{1}^{3}+m_{1}^{6}+m_{1}^{3}({{m}_{1}}+m_{1}^{2})=\frac{8{{h}^{3}}}{{{b}^{3}}}\] Again, from Eqs. (ii) and (iii), we get                 \[\left( \frac{a}{b} \right)+{{\left( \frac{a}{b} \right)}^{2}}+3\left( \frac{a}{b} \right)\left( \frac{-2h}{b} \right)=\frac{-8{{h}^{3}}}{{{b}^{3}}}\] \[\Rightarrow \]               \[{{a}^{2}}b+a{{b}^{2}}+8{{h}^{2}}=6abh\] On dividing both sides by\[abh\], we get                 \[\frac{a+b}{h}+\frac{8{{h}^{2}}}{ab}=6\]


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