diff --git a/.agents/skills/ofb/SKILL.md b/.agents/skills/ofb/SKILL.md
index f839c61..4ea0f34 100644
--- a/.agents/skills/ofb/SKILL.md
+++ b/.agents/skills/ofb/SKILL.md
@@ -135,6 +135,8 @@ Live chapters live under `content/{subject}/{slug}/`:
 
 **Math checkpoints / dots:** follow [math-chapter-patterns.md](math-chapter-patterns.md); shared dot classes in `content/math/subject.scss`.
 
+**Math equation layout (all subjects):** for chained calculations, **always prefer multi-line KaTeX** and **always align at the equal sign** using `aligned` (e.g. `\begin{aligned} ... &= ... \\ ... \end{aligned}`). Do not write long one-line chains; aligned multi-line steps are the default for readability and mobile layout.
+
 ### Interactive demo technology choices
 
 Interactive demos can use different technologies based on the learning goal and interaction type:
diff --git a/.agents/skills/ofb/math-chapter-patterns.md b/.agents/skills/ofb/math-chapter-patterns.md
index 4bb40a1..f77b3f0 100644
--- a/.agents/skills/ofb/math-chapter-patterns.md
+++ b/.agents/skills/ofb/math-chapter-patterns.md
@@ -7,6 +7,20 @@ Use these conventions in `core.html` and `supplement.html` so every math chapter
 - Wrapper: `<div class="book-prose">`
 - Section titles: `<h2 id="…" class="book-prose__heading">`
 - In-chapter links: plain `<a href="/math/…/">` inside `.book-prose` (theme styles them)
+- For chained calculations, prefer aligned multi-line KaTeX:
+
+```html
+<p class="book-formula">\[
+  \begin{aligned}
+    b &= \sqrt{c^2-a^2} \\
+      &= \sqrt{13^2-5^2} \\
+      &= \sqrt{144} \\
+      &= 12
+  \end{aligned}
+\]</p>
+```
+
+This keeps equal signs aligned and avoids long single-line equations on mobile.
 
 ## Checkpoints (required shape)
 
diff --git a/content/math/pythagoras-theorem/_index.md b/content/math/pythagoras-theorem/_index.md
index d59f9f2..ae3410e 100644
--- a/content/math/pythagoras-theorem/_index.md
+++ b/content/math/pythagoras-theorem/_index.md
@@ -54,31 +54,37 @@ strand = "Measures, Shape & Space"
   <h3 id="find-hypotenuse" class="book-prose__heading">Find the hypotenuse</h3>
   <p><strong>Example.</strong> If \(a=9\) and \(b=12\), find \(c\).</p>
   <p class="book-formula">\[
-    c = \sqrt{9^2 + 12^2}
-    = \sqrt{81 + 144}
-    = \sqrt{225}
-    = 15
+    \begin{aligned}
+      c &= \sqrt{9^2 + 12^2} \\
+        &= \sqrt{81 + 144} \\
+        &= \sqrt{225} \\
+        &= 15
+    \end{aligned}
   \]</p>
   <p>The third side is 15 ft.</p>
 
   <h3 id="find-leg" class="book-prose__heading">Find a missing leg</h3>
   <p><strong>Example.</strong> If \(c=13\) and \(a=5\), find \(b\).</p>
   <p class="book-formula">\[
-    b = \sqrt{c^2 - a^2}
-    = \sqrt{13^2 - 5^2}
-    = \sqrt{169 - 25}
-    = \sqrt{144}
-    = 12
+    \begin{aligned}
+      b &= \sqrt{c^2 - a^2} \\
+        &= \sqrt{13^2 - 5^2} \\
+        &= \sqrt{169 - 25} \\
+        &= \sqrt{144} \\
+        &= 12
+    \end{aligned}
   \]</p>
 
   <h3 id="radical-form" class="book-prose__heading">Leave answers in simplest radical form</h3>
   <p><strong>Example.</strong> If \(c=10\) and \(a=2\), find \(b\).</p>
   <p class="book-formula">\[
-    b = \sqrt{10^2 - 2^2}
-    = \sqrt{100 - 4}
-    = \sqrt{96}
-    = \sqrt{16 \cdot 6}
-    = 4\sqrt{6}
+    \begin{aligned}
+      b &= \sqrt{10^2 - 2^2} \\
+        &= \sqrt{100 - 4} \\
+        &= \sqrt{96} \\
+        &= \sqrt{16 \cdot 6} \\
+        &= 4\sqrt{6}
+    \end{aligned}
   \]</p>
 
   <h2 id="common-pitfall" class="book-prose__heading">Common pitfall</h2>
diff --git a/content/math/pythagoras-theorem/core.html b/content/math/pythagoras-theorem/core.html
index e9975f2..9101256 100644
--- a/content/math/pythagoras-theorem/core.html
+++ b/content/math/pythagoras-theorem/core.html
@@ -43,31 +43,37 @@
   <h3 id="find-hypotenuse" class="book-prose__heading">Find the hypotenuse</h3>
   <p><strong>Example.</strong> If \(a=9\) and \(b=12\), find \(c\).</p>
   <p class="book-formula">\[
-    c = \sqrt{9^2 + 12^2}
-    = \sqrt{81 + 144}
-    = \sqrt{225}
-    = 15
+    \begin{aligned}
+      c &= \sqrt{9^2 + 12^2} \\
+        &= \sqrt{81 + 144} \\
+        &= \sqrt{225} \\
+        &= 15
+    \end{aligned}
   \]</p>
   <p>The third side is 15 ft.</p>
 
   <h3 id="find-leg" class="book-prose__heading">Find a missing leg</h3>
   <p><strong>Example.</strong> If \(c=13\) and \(a=5\), find \(b\).</p>
   <p class="book-formula">\[
-    b = \sqrt{c^2 - a^2}
-    = \sqrt{13^2 - 5^2}
-    = \sqrt{169 - 25}
-    = \sqrt{144}
-    = 12
+    \begin{aligned}
+      b &= \sqrt{c^2 - a^2} \\
+        &= \sqrt{13^2 - 5^2} \\
+        &= \sqrt{169 - 25} \\
+        &= \sqrt{144} \\
+        &= 12
+    \end{aligned}
   \]</p>
 
   <h3 id="radical-form" class="book-prose__heading">Leave answers in simplest radical form</h3>
   <p><strong>Example.</strong> If \(c=10\) and \(a=2\), find \(b\).</p>
   <p class="book-formula">\[
-    b = \sqrt{10^2 - 2^2}
-    = \sqrt{100 - 4}
-    = \sqrt{96}
-    = \sqrt{16 \cdot 6}
-    = 4\sqrt{6}
+    \begin{aligned}
+      b &= \sqrt{10^2 - 2^2} \\
+        &= \sqrt{100 - 4} \\
+        &= \sqrt{96} \\
+        &= \sqrt{16 \cdot 6} \\
+        &= 4\sqrt{6}
+    \end{aligned}
   \]</p>
 
   <h2 id="common-pitfall" class="book-prose__heading">Common pitfall</h2>
diff --git a/static/feed.xml b/static/feed.xml
index b918d7b..1537bde 100644
--- a/static/feed.xml
+++ b/static/feed.xml
@@ -4,7 +4,21 @@
     <title>Open Free Books</title>
     <link>https://openfreebooks.org/</link>
     <description>Recent updates to Open Free Books pages and live chapters.</description>
-    <lastBuildDate>Thu, 21 May 2026 23:42:19 GMT</lastBuildDate>
+    <lastBuildDate>Fri, 22 May 2026 05:18:34 GMT</lastBuildDate>
+    <item>
+      <title>Coordinate geometry foundations</title>
+      <link>https://openfreebooks.org/math/coordinate-geometry-foundations/</link>
+      <guid isPermaLink="true">https://openfreebooks.org/math/coordinate-geometry-foundations/</guid>
+      <pubDate>Fri, 22 May 2026 05:18:34 GMT</pubDate>
+      <description>Plot points, find distances and midpoints, and work with slopes of lines in the Cartesian plane.</description>
+    </item>
+    <item>
+      <title>Pythagoras&apos; theorem</title>
+      <link>https://openfreebooks.org/math/pythagoras-theorem/</link>
+      <guid isPermaLink="true">https://openfreebooks.org/math/pythagoras-theorem/</guid>
+      <pubDate>Fri, 22 May 2026 05:18:34 GMT</pubDate>
+      <description>Use Pythagoras&apos; theorem and its converse in 2D problems and coordinate settings.</description>
+    </item>
     <item>
       <title>Basic math notation</title>
       <link>https://openfreebooks.org/math/basic-math-notation/</link>
@@ -89,13 +103,6 @@
       <pubDate>Thu, 21 May 2026 16:54:30 GMT</pubDate>
       <description>Find equations of circles, tangents, and intersections using coordinate methods and distance.</description>
     </item>
-    <item>
-      <title>Coordinate geometry foundations</title>
-      <link>https://openfreebooks.org/math/coordinate-geometry-foundations/</link>
-      <guid isPermaLink="true">https://openfreebooks.org/math/coordinate-geometry-foundations/</guid>
-      <pubDate>Thu, 21 May 2026 16:54:30 GMT</pubDate>
-      <description>Plot points, find distances and midpoints, and work with slopes of lines in the Cartesian plane.</description>
-    </item>
     <item>
       <title>Deductive geometry</title>
       <link>https://openfreebooks.org/math/deductive-geometry/</link>
@@ -145,13 +152,6 @@
       <pubDate>Thu, 21 May 2026 16:54:30 GMT</pubDate>
       <description>Count outcomes with permutations and combinations, then compute probabilities in finite sample spaces.</description>
     </item>
-    <item>
-      <title>Pythagoras&apos; theorem</title>
-      <link>https://openfreebooks.org/math/pythagoras-theorem/</link>
-      <guid isPermaLink="true">https://openfreebooks.org/math/pythagoras-theorem/</guid>
-      <pubDate>Thu, 21 May 2026 16:54:30 GMT</pubDate>
-      <description>Use Pythagoras&apos; theorem and its converse in 2D problems and coordinate settings.</description>
-    </item>
     <item>
       <title>Charts &amp; averages</title>
       <link>https://openfreebooks.org/math/statistical-charts-averages/</link>
diff --git a/static/sitemap.xml b/static/sitemap.xml
index 40fa4f5..23533f0 100644
--- a/static/sitemap.xml
+++ b/static/sitemap.xml
@@ -62,7 +62,7 @@
   </url>
   <url>
     <loc>https://openfreebooks.org/math/coordinate-geometry-foundations/</loc>
-    <lastmod>2026-05-21T16:54:30.000Z</lastmod>
+    <lastmod>2026-05-22T05:18:34.000Z</lastmod>
   </url>
   <url>
     <loc>https://openfreebooks.org/math/counting-and-numbers/</loc>
@@ -146,7 +146,7 @@
   </url>
   <url>
     <loc>https://openfreebooks.org/math/pythagoras-theorem/</loc>
-    <lastmod>2026-05-21T16:54:30.000Z</lastmod>
+    <lastmod>2026-05-22T05:18:34.000Z</lastmod>
   </url>
   <url>
     <loc>https://openfreebooks.org/math/quadratic-equations/</loc>