Update app.py

app.py

@@ -385,6 +385,10 @@ def create_demo():
                 "./examples/12.png",
                 "There is a numerical converter, the principle of which is shown in the following diagram: When the input x=16, the output y equals.",
             ],
+            [
+                "./examples/13.png",
+                "As shown in Figure 1, it is a right-angled triangular paper piece, $$ \angle A=30^{ \circ }$$, $$BC=\quantity{4}{cm}$$, it is folded so that point $$C$$ lands on point $$C'$$ on the hypotenuse, with the fold line being $$BD$$, as shown in Figure 2. Then, Figure 2 is folded along $$DE$$, so that point $$A$$ lands on point $$A'$$ on the extension of $$DC'$$, as shown in Figure 3. The length of the fold line $$DE$$ is ___.",
+            ],
         ]
         
         gr.Examples(