Updated sets.md

config/en/mkdocs.yaml

@@ -37,6 +37,9 @@ markdown_extensions:
   - pymdownx.mark
   - pymdownx.tilde
   - def_list
+  - admonition
+  - pymdownx.details
+  - pymdownx.superfences
 
 
 extra_javascript:

docs/en/mathematics/set-theory/sets.md

@@ -29,3 +29,7 @@ Let $B$ be set with $n$ elements. A subset $A$ of $B$ is completely determined b
 
 </details>
 <br>
+
+??? note "*Proof*:"
+
+    Let $B$ be set with $n$ elements. A subset $A$ of $B$ is completely determined by its elements. For each element $b \in B$ there are two options, it is in $A$ or it is not. So, there are $2^n$ options and thus $2^n$ different subsets $A$ of $B$.