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<h1 align="center">Fundamental Theorem of Algebra</h1>
<p>The &quot;Fundamental Theorem of Algebra&quot; is <b>not</b> the start of algebra or anything, but it does say something interesting about <a href="polynomials.html">polynomials</a>:</p>
<div class="center80">
<p align="center"><span class="larger">Any polynomial of degree <b>n</b> has <b>n</b> roots</span><br />
but we may need to use complex numbers</p>
</div>
<p>Let me explain: </p>
<div class="dotpoint">
<p>A <a href="polynomials.html">Polynomial</a> looks like this: </p>
<table border="0" align="center" cellpadding="5">
<tr align="center">
<td><img src="images/polynomial-1var-example.svg" alt="polynomial example" /></td>
</tr>
<tr align="center">
<td>example of a polynomial<br />
this one has 3 terms</td>
</tr>
</table>
</div>
<div class="dotpoint">
<p>The <a href="degree-expression.html">Degree</a> of a Polynomial with one variable is ...</p>
<p class="center">... the <a href="../exponent.html">largest exponent</a> of that variable.</p>
<p class="center"><img src="images/degree-example-a.svg" alt="polynomial" /></p>
</div>
<div class="dotpoint">
<p>A &quot;root&quot; (or &quot;zero&quot;) is where the <b>polynomial is equal to zero</b>.</p>
<p class="center"><img src="images/roots.svg" alt="roots (zeros)" /></p>
</div> <p>So, a polynomial of degree 3 will have 3 roots (places where the polynomial is equal to zero).
A polynomial of degree 4 will have 4 roots. And so on.</p>
<div class="example">
<h3>Example: what are the roots of <b>x<sup>2</sup> &minus; 9</b>?</h3>
<p><b>x<sup>2</sup> &minus; 9</b> has a degree of 2 (the largest exponent of x is 2), so there are 2 roots.</p>
<p>Let us solve it. We want it to be equal to zero: </p>
<div class="so"><b>x<sup>2</sup> &minus; 9 = 0</b></div>
<p>Add 9 to both sides:</p>
<div class="so"><b>x<sup>2</sup> = +9</b></div>
<p>Then take the square root of both sides:</p>
<div class="so"><b>x = &plusmn;3</b></div>
<p>So the roots are <b>&minus;3</b> and <b>+3</b></p>
<p class="center"><img src="images/x2-9.gif" width="178" height="229" alt="x^2 - 9" /></p>
</div>
<p>And there is something else of interest:</p>
<div class="dotpoint">
<p>A polynomial <b>can be rewritten like this</b>:</p>
</div>
<p align="center"><img src="images/polynomial-factoring.svg" alt="Polynomial Factoring" /></p>
<div class="words"> The factors like <span class="large">(x&minus;r<sub>1</sub>)</span> are called <b>Linear Factors</b>, because they make a <b>line</b> when we plot them. </div>
<div class="example">
<h3>Example: <b>x<sup>2</sup> &minus; 9</b></h3>
<p>The roots are <b>r<sub>1</sub> = &minus;3</b> and <b>r<sub>2</sub> = +3</b> (as we discovered above) so the factors are:</p>
<div class="so">x<sup>2</sup> &minus; 9 = <b>(x+3)(x&minus;3)</b></div>
<p>(in this case <b>a</b> is equal to <b>1</b> so I didn't put it in)</p>
<p align="center" class="larger">The Linear Factors are<b> (x+3)</b> and <b>(x&minus;3)</b></p>
</div>
<p>So knowing the <b>roots</b> means we also know the <b>factors</b>. </p>
<p>Here is another example:</p>
<div class="example">
<h3>Example: 3x<sup>2</sup> &minus; 12</h3>
<p> It is degree 2, so there are 2 roots.</p>
<p>Let us find the roots: We want it to be equal to zero:</p>
<div class="so">3x<sup>2</sup> &minus; 12 = 0</div>
<p>3 and 12 have a common factor of 3:</p>
<div class="so"> 3(x<sup>2</sup> &minus; 4) = 0</div>
<p>We can solve <b>x<sup>2</sup> &minus; 4</b> by moving the <b>&minus;4</b> to the right and taking square roots:</p>
<div class="so">x<sup>2</sup> = 4</div>
<div class="so">x = &plusmn;2 </div>
<p>So the roots are:</p>
<p align="center" class="large">x = &minus;2&nbsp; and&nbsp; x = +2</p>
<p>And so the factors are:</p>
<p align="center" class="large">3x<sup>2</sup> &minus; 12 = 3(x+2)(x&minus;2)</p>
</div>
<p>Likewise, when we know the <b>factors</b> of a polynomial we also know the <b>roots</b>.</p>
<div class="example">
<h3>Example: 3x<sup>2 </sup>&minus; 18x<sup> </sup>+ 24</h3>
<p>It is degree 2 so there are 2 factors.</p>
<p align="center"><span class="larger">3x<sup>2 </sup>&minus; 18x<sup> </sup>+ 24 = <span class="large">a(x&minus;r<sub>1</sub>)(x&minus;r<sub>2</sub>)</span></span></p>
<p>I just happen to know this is the factoring:</p>
<p align="center" class="larger">3x<sup>2 </sup>&minus; 18x<sup> </sup>+ 24 = <span class="large">3(x&minus;2<sub></sub>)(x&minus;4<sub></sub>)</span></p>
<p>And so the roots (zeros) are:</p>
<ul>
<li>+2</li>
<li>+4</li>
</ul>
<p>Let us check those roots:</p>
<p align="center">3(2)<sup>2 </sup>&minus; 18(2)<sup> </sup>+ 24 = 12 &minus; 36 + 24 = <b>0</b></p>
<p align="center">3(4)<sup>2 </sup>&minus; 18(4)<sup> </sup>+ 24 = 48 &minus; 72 + 24 = <b>0</b></p>
<p>Yes! The polynomial is zero at x = +2 and x = +4</p>
</div>
<h2>Complex Numbers</h2>
<p>We <b>may</b> need to use Complex Numbers to make the polynomial equal to zero.</p>
<div class="def">
<p align="center">A <a href="../numbers/complex-numbers.html">Complex Number</a> is a combination of a <a href="../numbers/real-numbers.html">Real Number</a> and an <a href="../numbers/imaginary-numbers.html">Imaginary Number</a></p>
</div>
<p align="center"><img src="../numbers/images/complex-number.svg" alt="Complex Number" /></p>
<p>And here is an example:</p>
<div class="example">
<h3>Example: x<sup>2</sup>&minus;x+1</h3>
<p>Can we make it equal to zero?</p>
<p align="center" class="large">x<sup>2</sup>&minus;x+1 = 0 </p>
<p>Using the <a href="../quadratic-equation-solver.html">Quadratic Equation Solver</a> the answer (to 3 decimal places) is:</p>
<div class="example2">
<table width="400" border="0" align="center">
<tr align="center">
<td class="larger">0.5 &minus; 0.866<b>i </b></td>
<td>and</td>
<td class="larger">0.5 + 0.866<b>i </b></td>
</tr>
</table>
</div>
<p>They are complex numbers! But they still work.</p>
<p>And so the factors are:</p>
<p align="center" class="larger">x<sup>2</sup>&minus;x+1 = ( x &minus; <span class="large">(0.5&minus;0.866<b>i </b>)</span> )( x &minus; <span class="large">(0.5+0.866<b>i </b>)</span> )</p>
</div>
<h2>Complex Pairs</h2>
<p>So the roots <span class="large">r<sub>1</sub>, r<sub>2</sub>, ... etc</span> may be Real or Complex Numbers.</p>
<p>But there is something interesting... </p>
<p align="center" class="larger">Complex Roots <b>always come in pairs</b>!</p>
<p align="center"><img src="../numbers/images/complex-conjugate-pair.svg" alt="Complex Conjugate Pairs" /></p>
<p>You saw that in our example above:</p>
<div class="example">
<h3>Example: x<sup>2</sup>&minus;x+1</h3>
<p>Has these roots:</p>
<div class="example2">
<table width="400" border="0" align="center">
<tr align="center">
<td class="larger">0.5 &minus; 0.866<b>i </b></td>
<td>and</td>
<td class="larger">0.5 + 0.866<b>i </b></td>
</tr>
</table>
</div>
</div>
<p>The pair are actually complex conjugates (where we <b>change the sign in the middle</b>) like this:</p>
<p align="center"><img src="../numbers/images/complex-conjugate.svg" alt="Complex Conjugate" /></p>
<p>Always in pairs? Yes (unless the polynomial has complex coefficients, but we are only looking at polynomials with real coefficients here!)</p>
<p>So we either get: </p>
<ul>
<li><b>no</b> complex roots</li>
<li><b>2</b> complex roots</li>
<li><b>4</b> complex roots,</li>
<li>etc</li>
</ul>
<p>And <b>never</b> 1, 3, 5, etc.</p>
<p>Which means we automatically know this:</p>
<div class="simple">
<table border="0" align="center">
<tr>
<th>Degree</th>
<th align="center">Roots</th>
<th align="center">Possible Combinations</th>
</tr>
<tr align="center">
<td>1</td>
<td>1</td>
<td> 1 Real Root </td>
</tr>
<tr align="center">
<td> 2 </td>
<td>2</td>
<td>2 Real Roots, <b>or</b> 2 Complex Roots </td>
</tr>
<tr align="center">
<td>3</td>
<td>3</td>
<td>3 Real Roots, <b>or</b> 1 Real and 2 Complex Roots</td>
</tr>
<tr align="center">
<td>4</td>
<td>4</td>
<td>4 Real Roots, <b>or</b> 2 Real and 2 Complex Roots, <b>or</b> 4 Complex Roots </td>
</tr>
<tr align="center">
<td>etc</td>
<td>&nbsp;</td>
<td>etc!</td>
</tr>
</table>
</div>
<p>And so:</p>
<p align="center"> When the degree is odd (1, 3, 5, etc) there is <b>at least one real root</b> ... guaranteed!</p>
<div class="example">
<h3>Example: 3x&minus;6</h3>
<p>The degree is 1. </p>
<p><b>There is one real root</b> </p>
<p>At +2 actually:</p>
<p align="center"><img src="images/graph-3xm6.gif" alt="3x-6" width="136" height="147" />: </p>
<p>You can actually see that it <b>must go through the x-axis</b> at some point.</p>
</div>
<h2>But Real is also Complex!</h2>
<p>I have been saying &quot;Real&quot; and &quot;Complex&quot;, but Complex Numbers do <b>include</b> the Real Numbers.</p>
<div class="center80">
<p>So when I say there are <i>&quot;2 Real, and 2 Complex Roots&quot;</i>, I should be saying something like <i>&quot;2 Purely Real (no Imaginary part), and 2 Complex (with a non-zero Imaginary Part) Roots&quot;</i> ...</p>
<p align="center">... but that is a lot of words that sound confusing ...</p>
<p align="center">... so I hope you don't mind my (perhaps too) simple language.</p>
</div>
<h2>Don't Want Complex Numbers?</h2>
<p>If we <b>don't</b> want Complex Numbers, we can multiply pairs of complex roots together:</p>
<div class="def">
<p align="center"><span class="larger">(a + b<b>i</b>)(a &minus; b<b>i</b>) = a<sup>2</sup></span><span class="larger"> + b<sup>2</sup></span></p>
</div>
<p>We get a <a href="quadratic-equation.html">Quadratic Equation</a> with no Complex Numbers ... it is purely Real.</p>
<div class="words">
<p>That type of Quadratic (where we can't &quot;reduce&quot; it any further without using Complex Numbers) is called an <b>Irreducible Quadratic</b>.</p>
</div>
<div class="words">
<p>And remember that simple factors like <span class="large">(x-r<sub>1</sub>)</span> are called <b>Linear Factors</b></p>
</div>
<p>&nbsp;</p>
<div class="center80">
<p>So a polynomial can be factored into all Real values using:</p>
<ul>
<li><b>Linear Factors</b>, and </li>
<li><b>Irreducible Quadratics</b></li>
</ul>
</div>
<p>&nbsp;</p>
<div class="example">
<h3>Example: x<sup>3</sup>&minus;1</h3>
<p align="center" class="larger">x<sup>3</sup>&minus;1 = (x&minus;1)(x<sup>2</sup>+x+1)</p>
<p>It has been factored into:</p>
<ul>
<li>1 linear factor: <span class="larger">(x&minus;1)</span></li>
<li>1 irreducible quadratic factor: <span class="larger">(x<sup>2</sup>+x+1)</span></li>
</ul>
<p>To factor <span class="larger">(x<sup>2</sup>+x+1)</span> further we need to use Complex Numbers, so it is an &quot;Irreducible Quadratic&quot;</p>
</div>
<h2>How do we know if the Quadratic is Irreducible?</h2>
<p>Just calculate the &quot;discriminant&quot;: <span class="time">b<sup>2</sup> - 4ac</span></p>
<p>(Read <a href="quadratic-equation.html">Quadratic Equations</a> to learn more about the discriminant.)</p>
<p align="center" class="larger">When <b>b<sup>2</sup> &minus; 4ac</b> is negative, the Quadratic has Complex solutions, <br />
and so is &quot;Irreducible&quot; </p>
<div class="example">
<h3>Example: 2x<sup>2</sup>+3x+5</h3>
<p>a = 2, b = 3, and c = 5:</p>
<p align="center"><b>b<sup>2</sup> &minus; 4ac</b> = 3<sup>2</sup> &minus; 4&times;2&times;5 = 9&minus;40 = <b>&minus;31</b></p>
<p>The discriminant is negative, so it is an &quot;Irreducible Quadratic&quot;</p>
</div>
<p>&nbsp;</p>
<h2>Multiplicity</h2>
<p>Sometimes a factor appears more than once. That is its <b>Multiplicity</b>.</p>
<div class="example">
<h3>Example: x<sup>2</sup>&minus;6x+9</h3>
<p align="center" class="larger">x<sup>2</sup>&minus;6x+9 = (x&minus;3)(x&minus;3)</p>
<p>&quot;(x&minus;3)&quot; appears twice, so the root &quot;3&quot; has <b>Multiplicity of 2</b></p>
</div>
<p>The <b>Multiplicities</b> are included when we say &quot;a polynomial of degree <b>n</b> has <b>n</b> roots&quot;.</p>
<div class="example">
<h3>Example: x<sup>4</sup>+x<sup>3</sup></h3>
<p>There <b>should be</b> 4 roots (and 4 factors), right?</p>
<p>Factoring is easy, just factor out <span class="larger">x<sup>3</sup></span>:</p>
<p align="center" class="large">x<sup>4</sup>+x<sup>3</sup> = x<sup>3</sup>(x+1) = x&middot;x&middot;x&middot;(x+1)</p>
<p align="center">there are 4 factors, with &quot;x&quot; appearing 3 times.</p>
<p>But there seem to be only 2 roots, at <b>x=&minus;1</b> and <b>x=0</b>:</p>
<p align="center"><img src="images/graph-x4px3.gif" alt="x^4+x^3" width="302" height="133" /></p>
<p>But counting Multiplicities there are actually 4:</p>
<ul>
<li>&quot;x&quot; appears three times, so the root &quot;0&quot; has a <b>Multiplicity of 3</b></li>
<li>&quot;x+1&quot; appears once, so the root &quot;&minus;1&quot; has a <b>Multiplicity of 1</b></li>
</ul>
<p align="center" class="larger">Total = 3+1 = 4</p>
</div>
<h2>Summary</h2>
<ul>
<div class="bigul">
<li>A polynomial of degree <b>n</b> has <b>n</b> roots (where the polynomial is zero)</li>
<li>A polynomial can be <b></b> factored like: <span class="beach"><i><b>a(x&minus;r<sub>1</sub>)(x&minus;r<sub>2</sub>)</b></i></span><i><b>... </b></i>where r<sub>1</sub>, etc are the roots</li>
<li>Roots may need to be <b>Complex Numbers</b></li>
<li>Complex Roots <b>always come in pairs</b></li>
<li>Multiplying a Complex pair gives an <b>Irreducible Quadratic</b></li>
<li>So a polynomial can be factored into all real factors which are either:
<ul>
<li><b>Linear Factors</b> or</li>
<li><b>Irreducible Quadratics</b></li>
</ul>
</li>
<li>Sometimes a factor appears more than once. That is its <b>Multiplicity</b>.</li>
</div>
</ul>
<p>&nbsp;</p>
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