{"id":10717,"date":"2017-11-02T11:49:16","date_gmt":"2017-11-02T18:49:16","guid":{"rendered":"https:\/\/magoosh.com\/hs\/?p=10717"},"modified":"2018-10-26T06:24:26","modified_gmt":"2018-10-26T13:24:26","slug":"ap-calculus-bc-review-ratio-root-tests","status":"publish","type":"post","link":"https:\/\/magoosh.com\/hs\/ap\/ap-calculus-bc-review-ratio-root-tests\/","title":{"rendered":"AP Calculus BC Review: Ratio and Root Tests"},"content":{"rendered":"<p>The Ratio and Root Tests are indispensable tools for finding out whether a series converges or diverges.  These tests also play a large role in determining the radius and interval of convergence for a series of functions.<\/p>\n<p>In this article, we will discuss how the Ratio and Root Tests work.  In addition, you&#8217;ll see a few examples from the AP Calculus BC exam.<\/p>\n<h2>The Ratio and Root Tests<\/h2>\n<p>Both the ratio and root tests are basically comparing how closely your series might be to a <a href=\"https:\/\/magoosh.com\/hs\/ap\/ap-calculus-bc-review-geometric-series\/\">geometric series<\/a>.<\/p>\n<p>Recall that a <strong>geometric series<\/strong> is a series in which the terms have a <strong>common ratio<\/strong>.<\/p>\n<p><img decoding=\"async\" src=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/geometric_series.gif\" alt=\"Geometric series\" width=\"213\" height=\"50\" class=\"aligncenter size-full wp-image-10899\" \/><\/p>\n<p><img decoding=\"async\" src=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/geom_series_2.gif\" alt=\"Geometric series written out\" width=\"252\" height=\"18\" class=\"aligncenter size-full wp-image-10909\" \/><\/p>\n<p>The test for convergence for a geometric series is very simple.  Let <em>r<\/em> be the common ratio.<\/p>\n<ul>\n<li>If |<em>r<\/em>| &lt; 1, then the geometric series converges.<\/li>\n<li>If |<em>r<\/em>| &ge; 1, then the geometric series diverges.<\/li>\n<\/ul>\n<figure id=\"attachment_11025\" aria-describedby=\"caption-attachment-11025\" style=\"width: 394px\" class=\"wp-caption aligncenter\"><img decoding=\"async\" src=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/Koch_Snowflake_Triangles.png\" alt=\"Koch Snowflake - Ratio and Root Tests\" width=\"394\" height=\"454\" class=\"size-full wp-image-11025\" srcset=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/Koch_Snowflake_Triangles.png 394w, https:\/\/magoosh.com\/hs\/files\/2017\/07\/Koch_Snowflake_Triangles-260x300.png 260w\" sizes=\"(max-width: 394px) 100vw, 394px\" \/><figcaption id=\"caption-attachment-11025\" class=\"wp-caption-text\">The area of fractals like the <a href=\"http:\/\/www.shodor.org\/interactivate\/activities\/KochSnowflake\/\" target=\"_blank\" rel=\"noopener noreferrer\">Koch Snowflake<\/a> can be found by evaluating a convergent geometric series.<\/figcaption><\/figure>\n<h3>The Ratio Test<\/h3>\n<p>Now suppose that you have a general infinite series like the one below.<\/p>\n<p><img decoding=\"async\" src=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/series.gif\" alt=\"series\" width=\"46\" height=\"51\" class=\"aligncenter size-full wp-image-10893\" srcset=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/series.gif 46w, https:\/\/magoosh.com\/hs\/files\/2017\/07\/series-27x30.gif 27w\" sizes=\"(max-width: 46px) 100vw, 46px\" \/><\/p>\n<p>A typical series will not have a common ratio.  In other words, if you divided <em>a<sub>n+1<\/sub><\/em> by <em>a<sub>n<\/sub><\/em>, the values you get would depend on <em>n<\/em>.  (In the case of a geometric series, <em>a<sub>n+1<\/sub><\/em> \/ <em>a<sub>n<\/sub><\/em> = <em>ar<sup>n+1<\/sup><\/em> \/ (<em>ar<sup>n<\/sup><\/em>) = <em>r<\/em>, which is a constant.)<\/p>\n<p>So, to get around the fact that there is no <em>common<\/em> ratio, what we do is to take the limit.  Then we will use that limiting value to decide whether our series converges or not.<\/p>\n<p>In fact, the Ratio Test determines <em>absolute convergence<\/em>.  (For more details, check out: <a href=\"https:\/\/magoosh.com\/hs\/ap\/ap-calculus-bc-review-absolute-conditional-convergence\/\">AP Calculus BC Review: Absolute and Conditional Convergence<\/a>.)<\/p>\n<p><strong>Ratio Test.<\/strong>  Let <img decoding=\"async\" src=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/limit_for_ratio_test.gif\" alt=\"limit for ratio test\" width=\"121\" height=\"46\" class=\"alignnone size-full wp-image-11024\" srcset=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/limit_for_ratio_test.gif 121w, https:\/\/magoosh.com\/hs\/files\/2017\/07\/limit_for_ratio_test-30x11.gif 30w\" sizes=\"(max-width: 121px) 100vw, 121px\" \/>.<\/p>\n<ul>\n<li>If <em>L<\/em> &lt; 1, then the series converges absolutely.<\/li>\n<li>If <em>L<\/em> &gt; 1, then the series diverges.<\/li>\n<li>However, if <em>L<\/em> = 1, then the ratio test is <strong>inconclusive<\/strong>. (And then other test(s) would be needed to determine whether this series converges or diverges.)<\/li>\n<\/ul>\n<h3>The Root Test<\/h3>\n<p>The Root Test also serves to isolate a potential common ratio, but does so in a different way.  This time, we take the <em>n<\/em>th root of the general term and then allow <em>n<\/em> to go to infinity.<\/p>\n<p>The inequalities are still the same, though.  We still have to test whether the limit is less, greater, or equal to 1.<\/p>\n<p><strong>Root Test.<\/strong>  Let <img decoding=\"async\" src=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/limit_for_root_test.gif\" alt=\"root test limit\" width=\"122\" height=\"28\" class=\"alignnone size-full wp-image-11028\" srcset=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/limit_for_root_test.gif 122w, https:\/\/magoosh.com\/hs\/files\/2017\/07\/limit_for_root_test-30x7.gif 30w\" sizes=\"(max-width: 122px) 100vw, 122px\" \/>.<\/p>\n<ul>\n<li>If <em>L<\/em> &lt; 1, then the series converges absolutely.<\/li>\n<li>If <em>L<\/em> &gt; 1, then the series diverges.<\/li>\n<li>However, if <em>L<\/em> = 1, then the ratio test is <strong>inconclusive<\/strong>. (And then other test(s) would be needed to determine whether this series converges or diverges.)<\/li>\n<\/ul>\n<p>So how is the <em>n<\/em>th root able to extract information about a potential common ratio?  Let&#8217;s take a look at how the limit behaves when you actually have a geometric series, &Sigma;<em>ar<sup>n<\/sup><\/em>.<\/p>\n<p><img decoding=\"async\" src=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/root_test_geometric.gif\" alt=\"Root test applied to geometric series\" width=\"143\" height=\"120\" class=\"aligncenter size-full wp-image-11030\" srcset=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/root_test_geometric.gif 143w, https:\/\/magoosh.com\/hs\/files\/2017\/07\/root_test_geometric-30x25.gif 30w\" sizes=\"(max-width: 143px) 100vw, 143px\" \/><\/p>\n<p>The limit is <em>L<\/em> = |<em>r<\/em>|, which is the common ratio! (Well, technically, it&#8217;s the absolute value of the common ratio, but close enough.)<\/p>\n<h4>Three Useful Limits for Root Test<\/h4>\n<p>It can be tricky to deal with a limit of the <em>n<\/em>th root of some quantity.  The following three properties will make your life a lot easier:<\/p>\n<p><img decoding=\"async\" src=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/nth_root_limits.gif\" alt=\"Three nth root limits\" width=\"277\" height=\"98\" class=\"aligncenter size-full wp-image-11031\" \/><\/p>\n<h3>Radius of Convergence<\/h3>\n<p>The Ratio and Root Tests can help to find the radius of convergence for a series of functions.  Here&#8217;s how it works:<\/p>\n<ol>\n<li>Set up the limit <em>L<\/em> involving the general terms, as outlined above.<\/li>\n<li>If <em>L<\/em> = 0, then the radius of convergence is infinite (<em>R<\/em> = &infin;).<\/li>\n<li>If <em>L<\/em> = &infin;, then the radius is zero (<em>R<\/em> = 0).<\/li>\n<li>Otherwise, if  <em>L<\/em> is nonzero and finite, then set up the inequality <em>L<\/em> &lt; 1, and solve for the expression |<em>x<\/em> &#8211; <em>c<\/em>|.  The numerical value that shows up on the right side of the inequality is equal to <em>R<\/em>.<\/li>\n<\/ol>\n<h2>Examples<\/h2>\n<h3>A Numerical Series<\/h3>\n<p>Determine whether the following series converges or diverges.<\/p>\n<p><img decoding=\"async\" src=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/series_exampleA.gif\" alt=\"Example A, Sum of 3n!\/(n^2 + 1)\" width=\"79\" height=\"50\" class=\"aligncenter size-full wp-image-11035\" srcset=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/series_exampleA.gif 79w, https:\/\/magoosh.com\/hs\/files\/2017\/07\/series_exampleA-30x19.gif 30w\" sizes=\"(max-width: 79px) 100vw, 79px\" \/><\/p>\n<h4>Your Intuition Might Deceive You<\/h4>\n<p>When it comes to sequences and series, often our intuition can be misleading.  For example, if you write out the first three terms of this series, you get:<\/p>\n<p>For <em>n<\/em> = 0, we have 3(0!)\/(0<sup>2<\/sup> + 1) = 3(1)\/1 = 3.<\/p>\n<p>Then for <em>n<\/em> = 1, we have 3(1!)\/(1<sup>2<\/sup> + 1) = 3(1)\/2 = 1.5.<\/p>\n<p>And for <em>n<\/em> = 2, the value is 3(2!)\/(2<sup>2<\/sup> + 1) = 3(2)\/5 = 1.2.<\/p>\n<p>It seems that the terms are just deceasing as <em>n<\/em> increases, right?  But the terms actually turn around and start increasing after a while!<\/p>\n<figure id=\"attachment_9567\" aria-describedby=\"caption-attachment-9567\" style=\"width: 640px\" class=\"wp-caption aligncenter\"><img decoding=\"async\" src=\"https:\/\/magoosh.com\/hs\/files\/2017\/03\/cat-633081_640.jpg\" alt=\"surprised cat\" width=\"640\" height=\"480\" class=\"size-full wp-image-9567\" srcset=\"https:\/\/magoosh.com\/hs\/files\/2017\/03\/cat-633081_640.jpg 640w, https:\/\/magoosh.com\/hs\/files\/2017\/03\/cat-633081_640-300x225.jpg 300w, https:\/\/magoosh.com\/hs\/files\/2017\/03\/cat-633081_640-600x450.jpg 600w, https:\/\/magoosh.com\/hs\/files\/2017\/03\/cat-633081_640-30x23.jpg 30w\" sizes=\"(max-width: 640px) 100vw, 640px\" \/><figcaption id=\"caption-attachment-9567\" class=\"wp-caption-text\">Surprised cat is surprised.<\/figcaption><\/figure>\n<p>For  <em>n<\/em> = 3, we have 3(3!)\/(3<sup>2<\/sup> + 1) = 3(6)\/10 = 1.8.<\/p>\n<p>Then for <em>n<\/em> = 4, the term is 3(4!)\/(4<sup>2<\/sup> + 1) = 3(24)\/17 = 4.235.<\/p>\n<p>In fact, the terms themselves become unbounded as <em>n<\/em> &rarr; &infin;.  That alone indicates that the series diverges (if you can prove it).  <\/p>\n<p>But let&#8217;s see how Ratio or Root Test works in this case.<\/p>\n<h4>Solution<\/h4>\n<p>Usually when there are factorials in the general term, the Ratio Test is easier to apply than the Root Test.<\/p>\n<p><img decoding=\"async\" src=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/series_exampleA_solutionA.gif\" alt=\"First part of the solution\" width=\"351\" height=\"277\" class=\"aligncenter size-full wp-image-11036\" \/><\/p>\n<p>Now, because the limit is greater than 1 &#8212; and really, how much greater than 1 could you get than infinity!? &#8212; we conclude that this series diverges.<\/p>\n<h3>When the Tests Fail<\/h3>\n<p>Determine whether the alternating harmonic series converges or diverges.<\/p>\n<p><img decoding=\"async\" src=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/alternating_harmonic.gif\" alt=\"Alternating harmonic series\" width=\"245\" height=\"95\" class=\"aligncenter size-full wp-image-10938\" \/><\/p>\n<h4>Solution<\/h4>\n<p>You can pick either the Ratio or Root Test.  I&#8217;ll set up the Ratio Test for you.<\/p>\n<p><img decoding=\"async\" src=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/series_exampleB_solutionA.gif\" alt=\"using ratio test for alternating harmonic series\" width=\"219\" height=\"158\" class=\"aligncenter size-full wp-image-11039\" \/><\/p>\n<p>This time, the limit is exactly 1.  That means that the Ratio Test is inconclusive.  It won&#8217;t help to try the Root Test instead &#8212; you&#8217;ll get the same result.<\/p>\n<p>So what do we do now?<\/p>\n<p>You&#8217;ll have to pick a different convergence test.  I&#8217;d recommend the Alternating Series Test (AST), because this series is alternating, after all.<\/p>\n<p>Examining the absolute value of the general term, 1\/<em>n<\/em>, we see that it does decrease down to zero (as <em>n<\/em> &rarr; &infin;).  Therefore, by the AST, the alternating harmonic series converges.<\/p>\n<h3>Finding the Radius of Convergence<\/h3>\n<p>Find the radius of convergence for the following series.<\/p>\n<p><img decoding=\"async\" src=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/series_exampleC.gif\" alt=\"Sum of 5n(3x + 2)^n\" width=\"120\" height=\"51\" class=\"aligncenter size-full wp-image-11032\" srcset=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/series_exampleC.gif 120w, https:\/\/magoosh.com\/hs\/files\/2017\/07\/series_exampleC-30x13.gif 30w\" sizes=\"(max-width: 120px) 100vw, 120px\" \/><\/p>\n<h4>Solution<\/h4>\n<p>This one is just begging for Root Test!<\/p>\n<p><img decoding=\"async\" src=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/series_exampleC_solutionA.gif\" alt=\"series example solution part A\" width=\"359\" height=\"83\" class=\"aligncenter size-full wp-image-11033\" \/><\/p>\n<p>Since the limit was neither 0 nor infinity, we have to set the expression less than 1 and solve.<\/p>\n<p><img decoding=\"async\" src=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/series_exampleC_solutionB.gif\" alt=\"Example solution, part B\" width=\"132\" height=\"121\" class=\"aligncenter size-full wp-image-11034\" srcset=\"https:\/\/magoosh.com\/hs\/files\/2017\/07\/series_exampleC_solutionB.gif 132w, https:\/\/magoosh.com\/hs\/files\/2017\/07\/series_exampleC_solutionB-30x28.gif 30w\" sizes=\"(max-width: 132px) 100vw, 132px\" \/><\/p>\n<p>The last line gives two pieces of information.  Both are derived from the general template, |<em>x<\/em> &#8211; <em>c<\/em>| &lt; <em>R<\/em>, where c is the center, and R is the radius of convergence.<\/p>\n<ul>\n<li>The radius of convergence is <em>R<\/em> = 1\/3.  <\/li>\n<li>The center of the power series is <em>c<\/em> = -2\/3.<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>The Ratio and Root Tests are indispensable tools for finding out whether a series converges or diverges. These tests also play a large role in determining the radius and interval of convergence for a series of functions. In this article, we will discuss how the Ratio and Root Tests work. In addition, you&#8217;ll see a [&hellip;]<\/p>\n","protected":false},"author":223,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[240],"tags":[241],"ppma_author":[24932],"class_list":["post-10717","post","type-post","status-publish","format-standard","hentry","category-ap","tag-ap-calculus"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO Premium plugin v21.7 (Yoast SEO v21.7) - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>AP Calculus BC Review: Ratio and Root Tests - Magoosh Blog | High School<\/title>\n<meta name=\"description\" content=\"The ratio and root tests are important for determining the convergence properties of series. 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