JEE Main & Advanced Mathematics Differentiation Question Bank Differentiation by Substitution

  • question_answer
    Let \[f(x)=\left| \begin{matrix}    {{x}^{3}} & \sin x & \cos x  \\    6 & -1 & 0  \\    p & {{p}^{2}} & {{p}^{3}}  \\ \end{matrix} \right|\], where p is a constant. Then \[\frac{{{d}^{3}}}{d{{x}^{3}}}\left\{ f(x) \right\}\]at \[x=0\]is           [IIT 1997 Cancelled]

    A)            p

    B)            \[p+{{p}^{2}}\]

    C)            \[p+{{p}^{3}}\]

    D)            Independent of p

    Correct Answer: D

    Solution :

               \[{f}'''(x)=\left| \,\begin{matrix}    \frac{{{d}^{3}}}{d{{x}^{3}}}{{x}^{3}} & \frac{{{d}^{3}}}{d{{x}^{3}}}\sin x & \frac{{{d}^{3}}}{d{{x}^{3}}}\cos x  \\    6 & -1 & 0  \\    p & {{p}^{2}} & {{p}^{3}}  \\ \end{matrix}\, \right|=\left| \,\begin{matrix}    6 & -\cos x & \sin x  \\    6 & -1 & 0  \\    p & {{p}^{2}} & {{p}^{3}}  \\ \end{matrix}\, \right|\]                    \[\therefore {f}'''(0)=\left| \,\begin{matrix}    6 & -1 & 0  \\    6 & -1 & 0  \\    p & {{p}^{2}} & {{p}^{3}}  \\ \end{matrix}\, \right|=0\], which is independent of p.


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