There are 1 questions in this calculation: for each question, the 3 derivative of x is calculated.
    Note that variables are case sensitive.
\[ \begin{equation}\begin{split}[1/1]Find\ the\ third\ derivative\ of\ function\ cos(\frac{{x}^{2}}{k})\ with\ respect\ to\ x：\\\end{split}\end{equation} \]\[ \begin{equation}\begin{split}\\Solution:&\\ &Primitive\ function\ = cos(\frac{x^{2}}{k})\\&\color{blue}{The\ first\ derivative\ function：}\\&\frac{d\left( cos(\frac{x^{2}}{k})\right)}{dx}\\=&\frac{-sin(\frac{x^{2}}{k})*2x}{k}\\=&\frac{-2xsin(\frac{x^{2}}{k})}{k}\\\\ &\color{blue}{The\ second\ derivative\ of\ function:} \\&\frac{d\left( \frac{-2xsin(\frac{x^{2}}{k})}{k}\right)}{dx}\\=&\frac{-2sin(\frac{x^{2}}{k})}{k} - \frac{2xcos(\frac{x^{2}}{k})*2x}{kk}\\=&\frac{-2sin(\frac{x^{2}}{k})}{k} - \frac{4x^{2}cos(\frac{x^{2}}{k})}{k^{2}}\\\\ &\color{blue}{The\ third\ derivative\ of\ function:} \\&\frac{d\left( \frac{-2sin(\frac{x^{2}}{k})}{k} - \frac{4x^{2}cos(\frac{x^{2}}{k})}{k^{2}}\right)}{dx}\\=&\frac{-2cos(\frac{x^{2}}{k})*2x}{kk} - \frac{4*2xcos(\frac{x^{2}}{k})}{k^{2}} - \frac{4x^{2}*-sin(\frac{x^{2}}{k})*2x}{k^{2}k}\\=&\frac{-12xcos(\frac{x^{2}}{k})}{k^{2}} + \frac{8x^{3}sin(\frac{x^{2}}{k})}{k^{3}}\\ \end{split}\end{equation} \]

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