{"id":10362,"date":"2017-06-22t10:50:57","date_gmt":"2017-06-22t17:50:57","guid":{"rendered":"\/\/www.catharsisit.com\/hs\/?p=10362"},"modified":"2017-06-18t16:00:56","modified_gmt":"2017-06-18t23:00:56","slug":"ap-calculus-bc-review-sequences-convergence","status":"publish","type":"post","link":"\/\/www.catharsisit.com\/hs\/ap\/ap-calculus-bc-review-sequences-convergence\/","title":{"rendered":"ap calculus bc review: sequences and convergence"},"content":{"rendered":"
one important topic that shows up on the ap calculus bc exam (but not on the ab) is sequences<\/em>. this review article is dedicated to sequences and their convergence properties.<\/p>\n basically, a sequence is just a list of numbers. <\/p>\n however, it’s also much more than that.<\/p>\n we usually study infinite sequences<\/strong>, those that go on forever according to some rule or pattern. furthermore, we are often interested in determining whether a sequence converges<\/strong> (that is, approaches some fixed value) or not.<\/p>\n a sequence<\/strong> is a list of (infinitely many) numbers, called the terms<\/strong> of the sequence.<\/p>\n there are a number of different ways to write a sequence.<\/p>\n when n<\/em> is unspecified, the expression a<\/em>n<\/sub><\/em> is called the general term<\/strong> of the sequence. <\/p>\n moreover, if we know that a<\/em>n<\/sub><\/em> = f<\/em>(n<\/em>) for some function f<\/em>, then we say that f<\/em>(n<\/em>) is the formula for the general term.<\/p>\n you have probably seen and worked with many different kinds of sequences already even if you didn’t call them sequences.<\/p>\n the fibonacci sequences is an example of a recursively-defined sequence<\/strong>, because we can write it by the following recursive rule. <\/p>\n by the way, the fibonacci sequence is important for many reasons, showing up in nature in the most unexpected ways.<\/p>\n we say that a sequence converges<\/strong> to a number a<\/em> if its terms get arbitrarily close to a<\/em> the further along in the sequence you get.<\/p>\n to be more precise, we say that the limit<\/strong> (as n<\/em> → ∞) of the convergent sequence exists (and equals a<\/em>). just like limits of functions, we use the “lim” notation.<\/p>\n the harmonic sequence (a<\/em>n<\/sub><\/em> = 1\/n<\/em>) converges to 0. how can we establish this fact? well intuitively speaking, if you plug in a very large value of n<\/em> into the formula 1\/n<\/em>, what do you get? a little experimentation may lead you to the guess that 1\/n<\/em> converges to 0.<\/p>\n if n<\/em> = 10, you get 1\/10 = 0.1, which is already pretty close to 0.<\/p>\n for n<\/em> = 100, you get 1\/100 = 0.01, even closer to 0.<\/p>\n for n<\/em> = 1,000,000, the value is 1\/(1,000,000) = 0.000001, very close to 0 indeed!<\/p>\n the higher n is, the closer 1\/n will get to 0. however to rigorously prove this requires a more careful argument. <\/p>\n for our purposes, we will simply state this limit fact without proof.<\/p>\n if the limit does not exist, then we say that the sequence diverges<\/strong> (or is divergent<\/strong>).<\/p>\n for example, the sequence of natural numbers, 1, 2, 3, 4, 5, …. is divergent because the values simply get larger without bound. there’s no limit<\/em> to the values, quite literally.<\/p>\n as you can see from the definition, testing the convergence of a sequence requires taking a limit.<\/p>\n there are a few standard tricks to working out these kinds of limits. the key is that the variable n<\/em> is tending toward infinity (∞), so most of the same techniques that worked to find horizontal asymptotes also work in this new setting.<\/p>\n by the way, now is a great time to review: how do you find the horizontal asymptotes of a function?<\/a><\/p>\n consider the sequence, (a) write out the first four terms of the sequence.<\/p>\n (b) determine whether the sequence converges. if it converges, then what is the limit?<\/p>\n (a) each term of a sequence can be found by plugging in n<\/em>-values, similar to how you might find the value of a function f<\/em>(x<\/em>) by plugging in x<\/em>-values.<\/p>\n thus the first four terms are: 3, 2.75, 2, 1.3125.<\/p>\n (b) we need to compute the limit as n<\/em> approaches infinity.<\/p>\n if you get stuck at this step, imagine how you would handle the problem if it involved x<\/em> instead of n<\/em>. for this one, i’d use l’hospital’s rule<\/a>.<\/p>\n the last line is based on the fact that the top of the fraction remains constant while the bottom grows without bound as n<\/em> → ∞.<\/p>\n furthermore, we can now say that the sequence converges (because the limit does exists), and it converges to 0.<\/p>\n now is a great time to remind you of a very important distinction in mathematical terminology. <\/p>\n there is a related concept called a series<\/strong>, which is by definition the sum of a sequence<\/em>.<\/p>\n in fact, you may already know one important fact. even though the harmonic sequence<\/em> converges (to the value 0), the harmonic series<\/em> actually diverges.<\/p>\n in other words, the sum of all of the reciprocals of natural numbers just grows and grows without bound.<\/p>\nwhat are sequences?<\/h2>\n
definition and notation<\/h3>\n
<\/p>\n
common sequences<\/h3>\n
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<\/p>\n
convergence and divergence<\/h2>\n
<\/p>\n
convergence — the harmonic sequence<\/h3>\n
.<\/p>\n
divergence — the natural number sequence<\/h3>\n
convergence and limits<\/h2>\n
example<\/h2>\n
.<\/p>\n
solution<\/h3>\n
<\/p>\n
<\/p>\n
sequences are not series<\/h2>\n
<\/p>\n