JEE Main & Advanced Mathematics Applications of Derivatives Question Bank Mock Test - Application of Derivatives

  • question_answer
    The lines tangent to the curves \[{{y}^{3}}-{{x}^{2}}y+5y-2x=0\]and \[{{x}^{4}}-{{x}^{3}}{{y}^{2}}+5x+2y=0\]at the origin intersect at an angles \[\theta \]equal to

    A) \[\frac{\pi }{6}\]            

    B) \[\frac{\pi }{4}\]

    C) \[\frac{\pi }{3}\]            

    D) \[\frac{\pi }{2}\]

    Correct Answer: D

    Solution :

    [d] Differentiating \[{{y}^{3}}-{{x}^{2}}y+5y-2x=0\]w.r.t. x, we get \[3{{y}^{2}}y'-2xy-{{x}^{2}}y'+5y'-2=0\] Or \[y'=\frac{2xy+2}{3{{y}^{2}}-{{x}^{2}}+5}\]or \[y{{'}_{(0,0)}}=\frac{2}{5}\] Differentiating \[{{x}^{4}}-{{x}^{3}}{{y}^{2}}+5x+2y=0\] w.r.t. x, we get \[4{{x}^{2}}-3{{x}^{2}}{{y}^{2}}-2{{x}^{3}}yy'+5+2y'=0\] Or \[y'=\frac{3{{x}^{2}}{{y}^{2}}-4{{x}^{3}}-5}{2-2{{x}^{3}}y}\]or \[y{{'}_{(0,0)}}=-\frac{5}{2}.\] Thus, both the curves intersect at right angle.


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