JEE Main & Advanced Mathematics Applications of Derivatives Question Bank Tangent and Normal

  • question_answer
    For the curve \[b{{y}^{2}}={{(x+a)}^{3}}\]the square of subtangent is proportional to     [RPET 1999]

    A)            \[{{\text{(Subnormal)}}^{1/2}}\]

    B)            Subnormal

    C)            \[{{\text{(Subnormal)}}^{\text{3/2}}}\]

    D)            None of these

    Correct Answer: B

    Solution :

               \[b{{y}^{2}}={{(x+a)}^{3}}\Rightarrow 2by.\frac{dy}{dx}=3{{(x+a)}^{2}}\Rightarrow \frac{dy}{dx}=\frac{3}{2by}{{(x+a)}^{2}}\]            \ Subnormal    = \[y\frac{dy}{dx}=\frac{3}{2b}{{(x+a)}^{2}}\]            \ Subtangent   = \[\frac{y}{\left( \frac{dy}{dx} \right)}=\frac{y}{\frac{3{{(x+a)}^{2}}}{2by}}=\frac{2b{{y}^{2}}}{3{{(x+a)}^{2}}}\]                             = \[\frac{2b{{\frac{(x+a)}{b}}^{3}}}{3{{(x+a)}^{2}}}=\frac{2}{3}(x+a)\]            \ (Subtangent)2 = \[\frac{4}{9}{{(x+a)}^{2}}\]            and \[\frac{{{(\text{Subtangent})}^{2}}}{\text{Subnormal}}=\frac{\frac{4}{9}{{(x+a)}^{2}}}{\frac{3}{2b}{{(x+a)}^{2}}}=\frac{8b}{27}\]            Þ (Subtangent)2 =  constant ´ (Subnormal).            \ (Subtangent)2 µ (Subnormal).


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