There are 1 questions in this calculation: for each question, the 1 derivative of x is calculated.
    Note that variables are case sensitive.
\[ \begin{equation}\begin{split}[1/1]Find\ the\ first\ derivative\ of\ function\ -kx(u(-x - L)u(x + L)) - kL(u(-x - L) - u(x - L))\ with\ respect\ to\ x：\\\end{split}\end{equation} \]\[ \begin{equation}\begin{split}\\Solution:&\\ &Primitive\ function\ = ku^{2}x^{3} + 2ku^{2}Lx^{2} + ku^{2}L^{2}x + 2kuLx\\&\color{blue}{The\ first\ derivative\ function：}\\&\frac{d\left( ku^{2}x^{3} + 2ku^{2}Lx^{2} + ku^{2}L^{2}x + 2kuLx\right)}{dx}\\=&ku^{2}*3x^{2} + 2ku^{2}L*2x + ku^{2}L^{2} + 2kuL\\=&3ku^{2}x^{2} + 4ku^{2}Lx + ku^{2}L^{2} + 2kuL\\ \end{split}\end{equation} \]

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