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<title>Regular Polygons - Properties</title>
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<h1 align="center">Properties of Regular Polygons</h1>
<h2>Polygon</h2>
<p>A <a href="polygons.html">polygon</a> is a <a href="plane.html">plane</a> shape (two-dimensional) with straight sides. Examples include triangles, quadrilaterals, pentagons, hexagons and so on.</p>
<h2>Regular</h2>
<table width="100%" border="0">
<tr>
<td>
<p>A &quot;Regular Polygon&quot; has:</p>
<ul>
<div class="bigul">
<li>all <b>sides</b> equal and </li>
<li>all <b>angles</b> equal. </li>
</div>
</ul>
<p>Otherwise it is<b> irregular</b>.</p>
</td>
<td>
<table border="0" align="center">
<tr align="center">
<td><b><img src="images/pentagon-regular.svg" alt="pentagon regular" /></b></td>
<td width="40">&nbsp;</td>
<td><b><img src="images/pentagon-irregular.svg" alt="irregular pentagon" /></b></td>
</tr>
<tr align="center">
<td>Regular Pentagon</td>
<td>&nbsp;</td>
<td>Irregular Pentagon</td>
</tr>
</table>
</td>
</tr>
</table>
<p>Here we look at <b>Regular Polygons</b> only.</p>
<h2>Properties</h2>
<p>So what can we know about regular polygons? First of all, we can work out angles.</p>
<table width="100%" border="0">
<tr>
<td><img src="images/exterior-angle.svg" alt="exterior angle" /></td>
<td>
<h2>Exterior Angle</h2>
<p>The <a href="exterior-angles-polygons.html">Exterior Angle</a> is the angle between any side of a shape,
<br /> and a line extended from the next side.</p>
</td>
</tr>
</table>
<div style="float:left; margin: 0 10px 5px 0;">
<script type="text/javascript">
exterioranglesMain();
</script>
</div>
<p>All&nbsp;the&nbsp;Exterior Angles of a polygon add up to 360&deg;, so:</p>
<p class="center larger">Each exterior angle must be <b>360&deg;/n </b></p>
<p>(where <b>n</b> is the number of sides)</p>
<p>&nbsp;</p>
<p>Press play button to see.</p>
<div style="clear:both"></div>
<div class="example">
<p style="float:right; margin: 10px 0 5px 10px;" class="center"><img src="images/external-angle.svg" alt="external angle of regular octagon" />
<br> <i>Exterior Angle<br>(of a regular octagon)</i></p>
<h3>Example: What is the exterior angle of a regular octagon?</h3>
<p>&nbsp;</p>
<p>An octagon has 8 sides, so:</p>
<div class="tbl">
<div class="row"><span class="left">Exterior angle =</span><span class="right">360&deg; / n</span></div>
<div class="row"><span class="left"> =</span><span class="right">360&deg; / 8</span></div>
<div class="row"><span class="left">=</span><span class="right">45&deg;</span></div>
</div>
</div>
<div style="clear:both"></div>
<table width="100%" border="0">
<tr>
<td><img src="images/exterior-interior-angle.gif" width="200" height="103" alt="exterior interior angle" /></td>
<td>
<h2>Interior Angles </h2>
<p>The <a href="interior-angles-polygons.html">Interior Angle</a> and Exterior Angle are measured from the same line, so they <b>add up to 180&deg;</b>.</p>
</td>
</tr>
</table>
<p class="center larger">Interior Angle = 180&deg; &minus; Exterior Angle</p>
<p>We know the<b> Exterior angle = 360&deg;/n</b>, so:</p>
<p class="center larger">Interior Angle = 180&deg; &minus; 360&deg;/n </p>
<div class="center80">
<p>Which can be rearranged like this:</p><div class="tbl">
<div class="row"><span class="left"> Interior Angle</span><span class="right">= 180&deg; &minus; 360&deg;/n</span></div>
<div class="row"><span class="left">&nbsp;</span><span class="right">= (n &times; 180&deg; / n) &minus; (2 &times; 180&deg; / n)</span></div>
<div class="row"><span class="left">&nbsp;</span><span class="right"> = (n&minus;2) &times; 180&deg;/n</span></div>
</div>
<p>So we also have this:</p>
<p class="center larger">Interior Angle = (n&minus;2) &times; 180&deg; / n</p>
</div>
<p>&nbsp;</p>
<div class="example">
<h3>Example: What is the interior angle of a regular octagon?</h3>
<p>A regular octagon has 8 sides, so:</p>
<p class="center">Exterior Angle = 360<b>&deg; </b>/ 8 = 45&deg;</p>
<p class="center"> Interior Angle = 180&deg; &minus; 45&deg; = <b>135&deg;</b> </p>
<p class="center"><img src="images/internal-angle.svg" alt="internal angle of regular octagon" /><i><br>
Interior Angle<br>
(of a regular octagon)</i></p>
<p>Or we could use: </p>
<div class="tbl">
<div class="row"><span class="left"> Interior Angle</span><span class="right">= (n&minus;2) &times; 180&deg; / n</span></div>
<div class="row"><span class="left">&nbsp;</span><span class="right"> = (8&minus;2) &times; 180&deg; / 8</span></div>
<div class="row"><span class="left">&nbsp;</span><span class="right"> = 6 &times; 180&deg; / 8</span></div>
<div class="row"><span class="left">&nbsp;</span><span class="right"> = 135&deg;</span></div>
</div>
</div>
<div class="example">
<h3>Example: What are the interior and exterior angles of a regular hexagon? </h3>
<p style="float:left; margin: 10px;"><img src="images/regular-hexagon.svg" alt="regular hexagon" /></p>
<p>A regular hexagon has 6 sides, so:</p>
<p class="center">Exterior Angle = 360<b>&deg; </b>/ 6 = 60&deg;</p>
<p class="center"> Interior Angle = 180<b>&deg; &minus; </b> 60&deg; = <b>120&deg;</b></p>
</div>
<br />
<p>And now for some names:</p>
<h2>&quot;Circumcircle, Incircle, Radius and Apothem ...&quot; </h2>
<p>Sounds quite musical if you repeat it a few times, but they are just the names of the &quot;outer&quot; and &quot;inner&quot; circles (and each radius) that can be drawn on a polygon like this:</p>
<p style="float:right; margin: 0 0 5px 10px;"><img src="images/apothem.svg" alt="apothem incircle, radius, circumcircle" /></p>
<p>&nbsp;</p>
<p>The &quot;outside&quot; circle is called a <b>circumcircle</b>, and it connects all vertices (corner points) of the polygon.</p>
<p>The radius of the circumcircle is also the <b>radius</b> of the polygon.</p>
<p>&nbsp;</p>
<p>The &quot;inside&quot; circle is called an <b>incircle</b> and it just touches each side of the polygon at its midpoint.</p>
<p>The radius of the incircle is the <b>apothem</b> of the polygon.</p>
<p>&nbsp;</p>
<p>(Not all polygons have those properties, but triangles and regular polygons do).</p>
<h2>Breaking into Triangles</h2>
<p style="float:left; margin: 0 10px 5px 0;"><img src="images/hexagon-triangles.svg" alt="hexagon triangles side and apothem" /></p>
<p>We can learn a lot about regular polygons by breaking them into triangles like this:</p>
<p>Notice that:</p>
<ul>
<li>the &quot;base&quot; of the triangle is one side of the polygon.</li>
<li>the &quot;height&quot; of the triangle is the &quot;Apothem&quot; of the polygon</li>
</ul>
<p>Now, the <a href="../area.html">area of a triangle</a> is half of the base times height, so:</p>
<p class="center">Area of one triangle = base &times; height / 2 = side &times; apothem / 2</p>
<p>To get the area of the whole polygon, just add up the areas of all the little triangles (&quot;n&quot; of them): </p>
<p class="center">Area of Polygon = <b>n</b> &times; side &times; apothem / 2</p>
<p>And since the perimeter is all the sides = n &times; side, we get: </p>
<p class="center larger">Area of Polygon = perimeter &times; apothem / 2</p>
<h2>A Smaller Triangle</h2>
<p>By cutting the triangle in half we get this:</p>
<p class="center"><img src="images/regular-polygon-sector.svg" alt="regular polygon sector" /><i><br>
<br>
(Note: The angles are in <a href="radians.html">radians</a>, not <a href="degrees.html">degrees</a>)</i></p>
<p>&nbsp;</p>
<p>The small triangle is right-angled and so we can use <a href="../sine-cosine-tangent.html">sine, cosine and tangent</a> to find how the <b>side</b>, <b>radius</b>, <b>apothem</b> and <b>n</b> (number of sides) are related:</p>
<div class="simple">
<table width="580" border="0" align="center" cellpadding="5">
<tr>
<td>sin(<font size="+1" face="Times New Roman, Times, serif">&pi;</font>/n) = (Side/2) / Radius</td>
<td><img src="../images/style/right-arrow.gif" width="46" height="46" alt="right arrow" /></td>
<td><b>Side = 2 &times; Radius &times; sin(<font size="+1" face="Times New Roman, Times, serif">&pi;</font>/n) </b></td>
</tr>
<tr>
<td>cos(<font size="+1" face="Times New Roman, Times, serif">&pi;</font>/n) = Apothem / Radius</td>
<td><img src="../images/style/right-arrow.gif" width="46" height="46" alt="right arrow" /></td>
<td><b>Apothem = Radius &times; cos(<font size="+1" face="Times New Roman, Times, serif">&pi;</font>/n) </b></td>
</tr>
<tr>
<td>tan(<font size="+1" face="Times New Roman, Times, serif">&pi;</font>/n) = (Side/2) / Apothem</td>
<td><img src="../images/style/right-arrow.gif" width="46" height="46" alt="right arrow" /></td>
<td><b>Side = 2 &times; Apothem &times; tan(<font size="+1" face="Times New Roman, Times, serif">&pi;</font>/n) </b></td>
</tr>
</table>
</div>
<p>There are a lot more relationships like those (most of them just &quot;re-arrangements&quot;), but those will do for now.</p>
<h2>More Area Formulas</h2>
<p>We can use that to calculate the area when we only know the Apothem: </p>
<div class="center80">
<div class="tbl">
<div class="row"><span class="left"><b>Area of Small Triangle</b></span><span class="right">= &frac12; &times; Apothem &times; (Side/2)</span></div>
</div>
<p>And we know (from the &quot;tan&quot; formula above) that:</p>
<p class="center"> <b>Side = 2 &times; Apothem &times; tan(<font size="+1" face="Times New Roman, Times, serif">&pi;</font>/n)</b></p>
<p>So:</p>
<div class="tbl">
<div class="row"><span class="left"><b>Area of Small Triangle</b></span><span class="right">= &frac12; &times; Apothem &times; (Apothem &times; tan(<font size="+1" face="Times New Roman, Times, serif">&pi;</font>/n))</span></div>
<div class="row"><span class="left">&nbsp;</span><span class="right"><b>= &frac12; &times; Apothem<sup>2</sup> &times; tan(<font size="+1" face="Times New Roman, Times, serif">&pi;</font>/n)</b></span></div>
</div>
</div>
<p>And there are 2 such triangles per side, or <b>2n</b> for the <b>whole polygon</b>:</p>
<p class="center larger">Area of Polygon = n &times; Apothem<sup>2</sup> &times; tan(<font size="+1" face="Times New Roman, Times, serif">&pi;</font>/n)</p>
<p>When we don't know the Apothem, we can use the same formula but re-worked for Radius or for Side:</p>
<p class="center larger">Area of Polygon = &frac12; &times; n &times; Radius<sup>2</sup> &times; sin(2 &times; <font size="+1" face="Times New Roman, Times, serif">&pi;</font>/n)</p>
<p class="center larger">Area of Polygon = &frac14; &times; n &times; Side<sup>2</sup> / tan(<font size="+1" face="Times New Roman, Times, serif">&pi;</font>/n)</p>
<h2>A Table of Values</h2>
<p>And here is a table of Side, Apothem and Area compared to a Radius of &quot;1&quot;, using the formulas we have worked out:</p>
<div class="simple">
<table border="0" align="center">
<tr align="center">
<th>Type</th>
<th>Name when
<br /> Regular</th>
<th>Sides
<br /> (n) </th>
<th>Shape</th>
<th>Interior Angle</th>
<th>Radius</th>
<th>Side</th>
<th>Apothem</th>
<th>Area</th>
</tr>
<tr align="center">
<td><a href="../triangle.html">Triangle</a>
<br /> <i>(or Trigon)</i></td>
<th>Equilateral
<br /> Triangle</th>
<td><b>3</b></td>
<td><img src="images/triangle-regular.svg" height="50" alt="regular triangle" /></td>
<td>60&deg;</td>
<td>1</td>
<td>1.732
<br /> (&radic;3)</td>
<td>0.5</td>
<td>1.299
<br /> (&frac34;&radic;3) </td>
</tr>
<tr align="center">
<td><a href="../quadrilaterals.html">Quadrilateral</a><i><br />
(or Tetragon)</i></td>
<th><a href="square.html">Square</a></th>
<td><b>4</b></td>
<td><img src="images/quadrilateral-regular.svg" height="50" alt="regular quadrilateral" /></td>
<td>90&deg;</td>
<td>1</td>
<td>1.414
<br /> (&radic;2) </td>
<td>0.707
<br /> (1/&radic;2) </td>
<td>2</td>
</tr>
<tr align="center">
<td><a href="pentagon.html">Pentagon</a></td>
<th>Regular
<br /> Pentagon</th>
<td><b>5</b></td>
<td><b><img src="images/pentagon-regular.svg" height="50" alt="pentagon regular" /></b></td>
<td>108&deg;</td>
<td>1</td>
<td>1.176</td>
<td>0.809</td>
<td>2.378</td>
</tr>
<tr align="center">
<td><a href="hexagon.html">Hexagon</a></td>
<th>Regular
<br /> Hexagon</th>
<td><b>6</b></td>
<td><b><img src="images/hexagon-regular.svg" alt="hexagon regular" height="50" /></b></td>
<td>120&deg;</td>
<td>1</td>
<td>1</td>
<td>0.866
<br /> (&frac12;&radic;3) </td>
<td>2.598
<br /> ((3/2)&radic;3)</td>
</tr>
<tr align="center">
<td>Heptagon
<br /> <i>(or Septagon)</i></td>
<th>Regular
<br /> Heptagon</th>
<td><b>7</b></td>
<td><b><img src="images/heptagon-regular.svg" alt="heptagon refular" height="50" /></b></td>
<td>128.571&deg;</td>
<td>1</td>
<td>0.868</td>
<td>0.901</td>
<td>2.736</td>
</tr>
<tr align="center">
<td><a href="octagon.html">Octagon</a></td>
<th>Regular
<br /> Octagon</th>
<td><b>8</b></td>
<td><b><img src="images/octagon-regular.svg" alt="octagon regular" height="50" /></b></td>
<td>135&deg;</td>
<td>1</td>
<td>0.765</td>
<td>0.924</td>
<td>2.828
<br /> (2&radic;2)</td>
</tr>
<tr align="center">
<td>...</td>
<th>...</th>
<td>&nbsp;</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
</tr>
<tr align="center">
<td>Pentacontagon</td>
<th>Regular
<br /> Pentacontagon</th>
<td><b>50</b></td>
<td>&nbsp;</td>
<td>172.8&deg;</td>
<td>1</td>
<td>0.126
<br /> </td>
<td>0.998
<br /> </td>
<td>3.133</td>
</tr>
<tr align="center">
<td colspan="9">(Note: values correct to 3 decimal places only)</td>
</tr>
</table>
</div>
<p>&nbsp;</p>
<p style="float:right; margin: 0 0 5px 10px;"><img src="images/regular-polygon-graph.svg" alt="regular polygon graph" /></p>
<h2>Graph</h2>
<p>And here is a graph of the table above, but with number of sides (&quot;n&quot;) from 3 to 30.</p>
<p>Notice that as &quot;n&quot; gets bigger, the Apothem is tending towards 1 (equal to the Radius) and that the Area is tending towards <b><font size="+1" face="Times New Roman, Times, serif">&pi;</font></b> = 3.14159..., just like a circle.</p>
<p>What is the Side length tending towards?</p>
<p>&nbsp;</p>
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