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<h1 class="center">Integration by Parts</h1>
<p>Integration by Parts is a special method of integration that is often useful when two functions are multiplied together, but is also helpful in other ways.</p>
<p>You will see plenty of examples soon, but first let us see the rule:</p>
<p class="center larger"><span class="intsymb"></span>u v dx = u<span class="intsymb"></span>v dx <span class="intsymb"></span>u' (<span class="intsymb"></span>v dx) dx</p>
<ul>
<li><b>u</b> is the function u(x)</li>
<li><b>v</b> is the function v(x)</li>
<li><b>u'</b> is the <a href="derivatives-rules.html">derivative</a> of the function u(x)</li>
</ul>
<p>The rule as a diagram:</p>
<p class="center"><img src="images/integral-parts-general.svg" alt="integration by parts general" height="181" width="311"></p>
<p>Let's get straight into an example:</p>
<div class="example">
<h3>Example: What is <span class="intsymb"></span>x cos(x) dx ?</h3>
<p>OK, we have <b>x</b> <i>multiplied by</i> <b>cos(x)</b>, so integration by parts is a good choice.</p>
<p>First choose which functions for <b>u</b> and <b>v</b>:</p>
<ul>
<li>u = x</li>
<li>v = cos(x)</li>
</ul>
<p class="large">So now it is in the format <span class="intsymb"><b></b></span><b>u v dx</b> we can proceed:</p>
<p>Differentiate <b>u</b>: <span class="larger">u' = x' = 1</span></p>
<p>Integrate <b>v</b>: <span class="larger"><span class="intsymb"></span>v dx = <span class="intsymb"></span>cos(x) dx = sin(x)</span> &nbsp; <i>(see <a href="integration-rules.html">Integration Rules</a>)</i></p>
<p>Now we can put it together:</p>
<p class="center"><img src="images/integral-parts-x-cosx.svg" alt="integration by parts x cos(x) dx" height="175" width="309"></p>
<p>Simplify and solve:</p>
<div class="so">x sin(x) <span class="intsymb"></span>sin(x) dx </div>
<div class="so">x sin(x) + cos(x) + C</div>
<p>Done!</p>
</div>
<p>&nbsp;</p>
<p>So we followed these steps:</p>
<ul>
<li>Choose u and v</li>
<li>Differentiate u: u'</li>
<li>Integrate v: <span class="intsymb"></span>v dx</li>
<li>Put u, u' and<span class="intsymb"></span>v dx into: <span class="center"><b>u<span class="intsymb"></span>v dx <span class="intsymb"></span>u' (<span class="intsymb"></span>v dx) dx</b></span></li>
<li>Simplify and solve</li>
</ul><br>
<div class="words">
<p>In English we can say that <b><span class="intsymb"></span>u v dx</b> becomes:</p>
<p class="center larger">(u integral v) minus integral of (derivative u, integral v)</p>
</div>
<p>&nbsp;</p>
<p>Let's try some more examples:</p>
<div class="example">
<h3>Example: What is<span class="intsymb"></span>ln(x)/x<sup>2</sup> dx ?</h3>
<p>First choose u and v:</p>
<ul>
<li>u = ln(x)</li>
<li>v = 1/x<sup>2</sup></li>
</ul>
<p>Differentiate u: <span class="larger">ln(x)' = <span class="intbl"><em>1</em><strong>x</strong></span></span></p>
<p>Integrate v: <span class="larger"><span class="intsymb"></span>1/x<sup>2</sup> dx = <span class="intsymb"></span>x<sup>-2</sup> dx = x<sup>-1</sup> = <span class="intbl"><em>1</em><strong>x</strong></span> </span> &nbsp;<i> (by the <a href="integration-rules.html">power rule</a>)</i></p>
<p>Now put it together:</p>
<p class="center"><img src="images/integral-parts-lnx-on-x2.svg" alt="integration by parts ln(x) on x^2" height="191" width="300"></p>
<p>Simplify:</p>
<div class="so">ln(x)/x <span class="larger"><span class="intsymb"></span></span><span class="larger">1/x<sup>2</sup></span> dx </div>
<div class="so">ln(x)/x 1/x + C</div>
<div class="so"> <span class="intbl"><em>ln(x) + 1</em><strong>x</strong></span> + C</div>
<!-- ln(x)+1/x + C -->
</div>
<p>&nbsp;</p><br>
<p><br></p>
<div class="example">
<h3>Example: What is <span class="intsymb"></span>ln(x) dx ?</h3>
<p>But there is only one function! How do we choose u and v ?</p>
<p>Hey! We can just choose v as being "1":</p>
<ul>
<li>u = ln(x)</li>
<li>v = 1</li>
</ul>
<p>Differentiate u: <span class="larger">ln(x)' = 1</span>/x</p>
<p>Integrate v: <span class="larger"><span class="intsymb"></span>1 dx = x </span></p>
<p>Now put it together:</p>
<p class="center"><img src="images/integral-parts-lnx.svg" alt="integration by parts ln(x)" height="185" width="303"></p>
<p>Simplify:</p>
<div class="so">x ln(x) <span class="larger"><span class="intsymb"></span></span>1<span class="larger"></span> dx </div>
<div class="so">x ln(x) x + C</div>
</div>
<p>&nbsp;</p>
<div class="example">
<h3>Example: What is<span class="intsymb"></span>e<sup>x</sup> x dx ?</h3>
<p>Choose u and v:</p>
<ul>
<li>u = e<sup>x</sup></li>
<li>v = x</li>
</ul>
<p>Differentiate u: <span class="larger">(e<sup>x</sup>)' = e<sup>x</sup></span></p>
<p>Integrate v: <span class="larger"> <span class="intsymb"></span>x dx = x<sup>2</sup>/2</span></p>
<p>Now put it together:</p>
<p class="center"><img src="images/integral-parts-ex-x.svg" alt="integration by parts e^x x" height="188" width="289"></p>
<p>
It only got worse!
</p>
</div>
<p>Well, that was a spectacular disaster.</p>
<p>Maybe we could choose a different u and v?</p>
<div class="example">
<h3>Example: <span class="intsymb"></span>e<sup>x</sup> x dx (continued)</h3>
<p>Choose u and v differently:</p>
<ul>
<li>u = x</li>
<li>v = e<sup>x</sup></li>
</ul>
<p>Differentiate u: <span class="larger">(x)' = 1</span></p>
<p>Integrate v: <span class="larger"><span class="intsymb"></span>e<sup>x</sup> dx = e<sup>x</sup></span></p>
<p>Now put it together:</p>
<p class="center"><img src="images/integral-parts-x-ex.svg" alt="integration by parts x e^x" height="175" width="285"></p>
<p>Simplify:</p>
<div class="so"> x <span class="larger">e<sup>x</sup></span> <span class="larger">e<sup>x</sup></span> + C </div>
<div class="so"><span class="larger">e<sup>x</sup></span>(x1) + C</div>
</div>
<p>The moral of the story: Choose <b>u</b> and <b>v</b> carefully!</p>
<div class="center80">
<p>Choose a <b>u</b> that gets simpler when you differentiate it and a <b>v</b> that doesn't get any more complicated when you integrate it.</p>
</div>
<p>A helpful rule of thumb is <span class="large">I LATE</span>. Choose <b>u</b> based on which of these comes first:</p>
<ul>
<li><b>I</b>: <a href="../algebra/trig-inverse-sin-cos-tan.html">Inverse trigonometric functions</a> such as sin<sup>-1</sup>(x), cos<sup>-1</sup>(x), tan<sup>-1</sup>(x)</li>
<li><b>L</b>: <a href="../sets/function-logarithmic.html">Logarithmic</a> functions such as ln(x), log(x)</li>
<li><b>A</b>: <a href="../numbers/algebraic-numbers.html">Algebraic</a> functions such as x<sup>2</sup>, x<sup>3</sup></li>
<li><b>T</b>: <a href="../sine-cosine-tangent.html">Trigonometric functions</a> such as sin(x), cos(x), tan (x)</li>
<li><b>E</b>: <a href="../sets/function-exponential.html">Exponential functions</a> such as e<sup>x</sup>, 3<sup>x</sup></li>
</ul><br><p>And here is one last (and tricky) example:</p>
<div class="example">
<h3>Example: <span class="intsymb"></span>e<sup>x</sup> sin(x) dx</h3>
<p>Choose u and v:</p>
<ul>
<li>u = sin(x)</li>
<li>v = e<sup>x</sup></li>
</ul>
<p>Differentiate u: <span class="larger">sin(x)' = cos(x)</span></p>
<p>Integrate v: <span class="larger"><span class="intsymb"></span>e<sup>x</sup> dx = e<sup>x</sup></span></p>
<p>Now put it together:</p>
<div class="so"><span class="intsymb"></span>e<sup>x</sup> sin(x) dx = sin(x) e<sup>x</sup> <span class="intsymb"></span>cos(x) e<sup>x</sup> dx</div>
<p>&nbsp;</p>
<p>It looks worse, but let us persist! To find <span class="large"><span class="intsymb"></span>cos(x) e<sup>x</sup> dx</span> we can use integration by parts <b>again</b>:</p>
<p>Choose u and v:</p>
<ul>
<li>u = cos(x)</li>
<li>v = e<sup>x</sup></li>
</ul>
<p>Differentiate u: <span class="larger">cos(x)' = -sin(x)</span></p>
<p>Integrate v: <span class="larger"><span class="intsymb"></span>e<sup>x</sup> dx = e<sup>x</sup></span></p>
<p>Now put it together:</p>
<div class="so"><span class="intsymb"></span>e<sup>x</sup> sin(x) dx = sin(x) e<sup>x</sup> (cos(x) e<sup>x</sup> <span class="intsymb"></span>sin(x) e<sup>x</sup> dx)</div>
<p>Simplify:</p>
<div class="so"><span class="intsymb"></span>e<sup>x</sup> sin(x) dx = e<sup>x</sup> sin(x) e<sup>x</sup> cos(x) <span class="intsymb"></span> e<sup>x</sup> sin(x)dx</div>
<p>Now we have the <b>same integral on both sides</b> (except one is subtracted) ...</p>
<p>... so we can bring the right hand integral over to the left and we get:</p>
<div class="so">2<span class="intsymb"></span>e<sup>x</sup> sin(x) dx = e<sup>x</sup> sin(x) e<sup>x</sup> cos(x)</div>
<p>Simplify:</p>
<div class="so"><span class="intsymb"></span>e<sup>x</sup> sin(x) dx = ½ e<sup>x</sup> (sin(x) cos(x)) + C</div>
</div>
<p>&nbsp;</p>
<h2>Definite Integrals</h2>
<p>When the integral has an interval like [a, b] we can use either of these:</p><span style="font-size:140%;"></span>
<div class="center larger">
<div class="intgl">
<div class="to">b</div>
<div class="symb"></div>
<div class="from">a</div>
</div>u v dx =
<span style="display:inline-block; transform: scaleY(2);">[</span>
u<div class="intgl">
<div class="to">&nbsp;</div>
<div class="symb"></div>
<div class="from">&nbsp;</div>
</div>v dx
<div class="intgl">
<div class="to">&nbsp;</div>
<div class="symb"></div>
<div class="from">&nbsp;</div>
</div>u'(<div class="intgl">
<div class="to">&nbsp;</div>
<div class="symb"></div>
<div class="from">&nbsp;</div>
</div>v dx) dx
<span style="display:inline-block; transform: scaleY(2);">]</span>
<div style="display:inline-block; font: 0.8em Verdana; text-align:center;transform: translateY(30%) translateX(-30%);">
<div>b</div><br>
<div>a</div>
</div>
</div>
<div class="center larger">
<div class="intgl">
<div class="to">b</div>
<div class="symb"></div>
<div class="from">a</div>
</div>u v dx =
<span style="display:inline-block; transform: scaleY(2);">[</span>
u<div class="intgl">
<div class="to">&nbsp;</div>
<div class="symb"></div>
<div class="from">&nbsp;</div>
</div>v dx
<span style="display:inline-block; transform: scaleY(2);">]</span>
<div style="display:inline-block; font: 0.8em Verdana; text-align:center;transform: translateY(30%) translateX(-30%);">
<div>b</div><br>
<div>a</div>
</div>
<div class="intgl">
<div class="to">b</div>
<div class="symb"></div>
<div class="from">a</div>
</div>u'(<div class="intgl">
<div class="to">&nbsp;</div>
<div class="symb"></div>
<div class="from">&nbsp;</div>
</div>v dx) dx</div>
<p>Where u and v are functions of x, and a and b are the limits on x. </p>The second version can help us see the relationship between the left and right integrals.<br>
<p>See <a href="integration-definite.html">Definite Integrals</a> for more info.</p>
<span style="font-size:140%;"></span>
<p></p>
<h2>Footnote: Where Did "<span class="center">Integration by Parts</span>" Come From?</h2>
<div class="center80">
<p>It is based on the <a href="derivatives-rules.html">Product Rule for Derivatives</a>:</p>
<div class="so">(uv)' = uv' + u'v</div>
<p>Integrate both sides and rearrange:</p>
<div class="so"><span class="intsymb"></span>(uv)' dx = <span class="intsymb"></span>uv' dx + <span class="intsymb"></span>u'v dx</div>
<div class="so">uv = <span class="intsymb"></span>uv' dx + <span class="intsymb"></span>u'v dx</div>
<div class="so"><span class="intsymb"></span>uv' dx = uv <span class="intsymb"></span>u'v dx</div>
<p>Some people prefer that last form, but I like to replace <b>v' with w</b> and <b>v with<span class="intsymb"></span>w dx</b> which makes the left side simpler:</p>
<div class="so"><span class="intsymb"></span>uw dx = u<span class="intsymb"></span>w dx <span class="intsymb"></span>u'(<span class="intsymb"></span>w dx) dx</div>
</div>
<p>&nbsp;</p>
<div class="questions">6844, 6845, 6846, 6847, 6848, 6849, 6850, 6851, 6852, 6853</div>
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<a href="integration-rules.html">Integration Rules</a>
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