Update textbook.fr.md

00.brainstorming-pedagogical-teams/40.collection-existing-pedagogical-content/04.reference-frames-coordinate-systems/textbook.fr.md

@@ -662,6 +662,17 @@ the product of their norms. And the volume defined by these 3 vectors is not the
 of their norms.
 
 
+* **N3 ($`\rightarrow`$ N4)**<br>
+<br>http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=102-05-06.<br>
+[ES] Según la dirección elegida, los **elementos escalares de superficie $`dA`$** 
+en coordenadas cilíndricas son :<br>
+[FR] Selon la direction choisie, les **éléments scalaires de surface $`dA`$** en
+coordonnées cylindriques sont :<br>
+[EN] According to the chosen direction, the **scalar surface elements $`dA`$** in 
+cylindrical coordinates are :<br>
+<br>$`dA_{\rho\varphi}=dl_{\rho}\;dl\varphi=d\rho\cdot\rho\;d\varphi\quad`$, 
+$`\quad dA_{\rho z}=dl_{\rho}\;dlz=d\rho\;dz\quad`$, $`\quad dA_{\varphi z}
+=dl_{\varphi}\;dlz=\rho\,d\varphi\;dz`$<br>