Update textbook.fr.md

12.temporary_ins/40.classical-mechanics/30.n3/07.coherence/textbook.fr.md

@@ -505,7 +505,7 @@ $`\overrightarrow{e_{\rho}}=\cos\theta\overrightarrow{e_x}+\sin\theta\overrighta
 $`\overrightarrow{e_{\theta}}=-\sin\theta\overrightarrow{e_x}+\cos\theta\overrightarrow{e_z}`$
 
 $`\begin{align}
-\dfrac{d\overrightarrow{e_{\rho}}}{dt}&=\dfrac{d}{dt\big(\cos\theta\overrightarrow{e_x}+\sin\theta\overrightarrow{e_z}\big)\\
+\dfrac{d\overrightarrow{e_{\rho}}}{dt}&=\dfrac{d}{dt}\big(\cos\theta\overrightarrow{e_x}+\sin\theta\overrightarrow{e_z}\big)\\
 \\
 &=\left[\dfrac{d\cos\theta}{dt}\overrightarrow{e_x}+\sin\theta\dfrac{d\overrightarrow{e_z}}{dt}\right]
 +\left[\dfrac{d(-\sin\theta)}{dt}\overrightarrow{e_z}+\cos\theta\dfrac{d\overrightarrow{e_z}}{dt}\right]\\