JCECE Engineering JCECE Engineering Solved Paper-2006

  • question_answer
    The two curves \[{{x}^{3}}-3x{{y}^{2}}+2=0\] and\[3{{x}^{2}}y-{{y}^{3}}-2=0\]

    A)  cut at right angles

    B)  touch each other

    C)  cut at an angle\[\frac{\pi }{3}\]

    D)  cut at an angle\[\frac{\pi }{4}\]

    Correct Answer: A

    Solution :

    Key Idea: The angle between the curves is the angle between the tangents to the curve. We have\[,\]     \[{{x}^{3}}-3x{{y}^{2}}+2=0\]      ... (i) and                        \[3{{x}^{2}}y-{{y}^{3}}-2=0\]       ... (ii) On differentiating Eq. (i) and (ii) with respect to\[x\], we get                 \[{{\left( \frac{dy}{dx} \right)}_{{{c}_{1}}}}={{m}_{1}}\frac{{{x}^{2}}-{{y}^{2}}}{xy}\] and        \[{{\left( \frac{dy}{dx} \right)}_{{{c}_{2}}}}={{m}_{2}}=\frac{-2xy}{{{x}^{2}}-{{y}^{2}}}\] Since     \[{{m}_{1}}\times {{m}_{2}}=-1\] Hence, the two curves cut at right angles.


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