Bonjour Zoulou29
[tex][\dfrac{1}{EI} ( \dfrac{P}{12} \times x^3- \dfrac{PL^2}{16} \times x)]'=\dfrac{1}{EI} ( \dfrac{P}{12} \times 3x^2- \dfrac{PL^2}{16} \times 1)\\\\\\=\dfrac{1}{EI} ( \dfrac{3Px^2}{12} - \dfrac{PL^2}{16})\\\\\\=\dfrac{1}{EI} ( \dfrac{Px^2}{4} - \dfrac{PL^2}{16})\\\\\\=\dfrac{P}{EI} ( \dfrac{x^2}{4} - \dfrac{L^2}{16})\\\\\\=\dfrac{P}{EI} ( \dfrac{4x^2}{16} - \dfrac{L^2}{16})\\\\\\=\boxed{\dfrac{P}{16EI} (4x^2-L^2)}[/tex]