JCECE Engineering JCECE Engineering Solved Paper-2015

  • question_answer
    If the slopes of the lines given by\[a{{x}^{2}}+2hxy+b{{y}^{2}}=0\]are in the ratio \[3:1\], then\[{{h}^{2}}\] is equal to

    A) \[\frac{ab}{3}\]                                

    B) \[\frac{4ab}{3}\]

    C)  \[\frac{4a}{3b}\]                                             

    D)   None of these

    Correct Answer: B

    Solution :

    Let \[y={{m}_{1}}x\] and \[y={{m}_{2}}x\] be the lines represented by the given equation. Then,                 \[{{m}_{1}}+{{m}_{2}}=\frac{-2h}{b}\]and\[{{m}_{1}}{{m}_{2}}=\frac{a}{b}\] We have,             \[{{m}_{1}}:{{m}_{2}}=3:1\] \[\Rightarrow \]               \[{{m}_{1}}=3{{m}_{2}}\] \[\because \]     \[{{m}_{1}}+{{m}_{2}}=\frac{-2h}{b}\]and\[{{m}_{1}}{{m}_{2}}=\frac{a}{b}\] \[\therefore \]  \[4{{m}_{2}}=-\frac{2h}{b}\]and\[3m_{2}^{2}=\frac{a}{b}\] On solving above equations, we get                 \[3{{\left( -\frac{h}{2b} \right)}^{2}}=\frac{a}{b}\] \[\Rightarrow \]               \[{{h}^{2}}=\frac{4ab}{3}\]


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