Update cheatsheet.fr.md

12.temporary_ins/10.electrostatics-vacuum/20.causes-stationary-electric-field/20.overview/cheatsheet.fr.md

@@ -449,7 +449,7 @@ $`\quad=\quad\dfrac{\dens^{1D}\cdot R\,d\varphi}{4\pi\epsilon_0}\cdot\dfrac{1}{d
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   avec    
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-  **$`\displaystyle E_M=\int_{\varphi = 0}^{2\pi}\dfrac{\dens^{1D}\; R}{4\pi\epsilon_0}\cdot\dfrac{1}{d^2}\,\cos(\varphi)\;d\varphi`$**
+  **$`\displaystyle E_M=\int_{\varphi = 0}^{2\pi}\dfrac{\dens^{1D}\; R}{4\pi\epsilon_0}\cdot\dfrac{1}{d^2}\,\cos(\alpha)\;d\varphi`$**
 
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