Update cheatsheet.fr.md

12.temporary_ins/40.classical-mechanics/20.n2/10.framework-of-classical-mechanics-2/20.overview/cheatsheet.fr.md

@@ -254,6 +254,23 @@ sur les condtions de mesure des durées et des longueurs?
 <span style="font-family:'Times New Roman';font-weight:bold;font-size:0.85rem;color:#6699ff"> entre ces deux instants.</span></p>
 <hr width=100% size=6px border-style="solid" color=#00bb33>
 
+<br><br>
+
+<p style="font-family:'Times New Roman';font-weight:bold;font-size:1.15rem;text-align:left;color:#00bb33">Tous les observateurs partagent</p>
+<hr width=100% size=6px border-style="solid" color=#00bb33>
+<p style="font-family:'Times New Roman';font-weight:bold;font-size:1.15rem;text-align:left;color:#cccccc">un <span style="color:#0066cc"> invariant spatial<br>
+<span style="font-family:'Times New Roman';margin-left:30px;font-weight:bold;font-size:0.85rem;text-align:right;color:#6699ff">entre deux points A et B de l'espace </span><br>
+<span style="font-family:'Times New Roman';font-weight:bold;font-size:1.15rem;color:#cccccc">appelé 
+</span><span style="font-family:'Times New Roman';font-weight:bold;font-size:1.15rem;left;color:#cc3300"> longueur  l<sub>AB</sub>
+</span><span style="font-family:'Times New Roman';font-weight:bold;font-size:0.85rem;color:#6699ff"> entre ces deux points.</span></p>
+<hr width=100% size=6px border-style="solid" color=#00bb33>
+<p style="font-family:'Times New Roman';font-weight:bold;font-size:1.15rem;text-align:left;color:#cccccc">un <span style="color:#0066cc"> invariant temporel<br>
+<span style="font-family:'Times New Roman';margin-left:30px;font-weight:bold;font-size:0.85rem;text-align:right;color:#6699ff">entre deux instants t<sub>1</sub> et t<sub>2</sub> du temps</span><br>
+<span style="font-family:'Times New Roman';font-weight:bold;font-size:1.15rem;color:#cccccc">appelé 
+</span><span style="font-family:'Times New Roman';font-weight:bold;font-size:1.15rem;left;color:#993300"> durée  t<sub>2</sub> - t<sub>1</sub></span>
+<span style="font-family:'Times New Roman';font-weight:bold;font-size:0.85rem;color:#6699ff"> entre ces deux instants.</span></p>
+<hr width=100% size=6px border-style="solid" color=#00bb33>
+
 
    ![](https://m3p2.com/temporary_ins/relativity/n2/an-euclidian-space-time/overview/train-speed-0-persons-ABD_L1200.jpg)