diff --git a/academic/misc/2024-math-calendar/12-dec.html b/academic/misc/2024-math-calendar/12-dec.html
index b23e0c1..067dfd3 100644
--- a/academic/misc/2024-math-calendar/12-dec.html
+++ b/academic/misc/2024-math-calendar/12-dec.html
@@ -950,6 +950,24 @@ <h3 id="dec17">Dec 17</h3>
 
 <h3 id="dec18">Dec 18</h3>
 
+<svg version="1.1" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 10 5" width="200" height="100" class="img_center">
+    <g stroke="black" stroke-width="0.05" fill="none">
+        <line x1="1" x2="9" y1="4" y2="4" />
+        <line x1="1" x2="5" y1="1" y2="4" />
+        <path d="M 5.75 4 A 0.75 0.75 0 0 0 4.4 3.55" />
+        <path d="M 4 4 A 1 1 0 0 1 4.2 3.4" />
+    </g>
+    <g fill="black" font-size="0.05em">
+        <text x="2" y="3.5">4x</text>
+        <text x="6" y="3.5">6x</text>
+    </g>
+</svg>
+
+<p>
+These are supplementary angles so
+\(4x+6x=180\Rightarrow10x=180\Rightarrow x=18\)
+</p>
+
 <h3 id="dec19">Dec 19</h3>
 
 <h3 id="dec20">Dec 20</h3>