1 Table of Contents Introduction…………………………………………………………………………………………………………2 Examples 1 and 2: Conceptual and Graphical……………………………………………………2-5 Definition of Maclaurin and Taylor Series; common Maclaurin Series……………….5-6 Example 3: Analytical………………………………………………………………………………………..6-8 Example 4: AP Level Multiple Choice……………………………………………………………………8 Example 5: AP Level Free Response………………………………………………………………..9-10 1 2 Example 6: Approximating ∫0 π π₯ ……………………………………………………………………10-11 Biography of Colin Maclaurin……………………………………………………………………………..12 Applications of Taylor and Maclaurin Series……………………………………………………….13 Works Cited………………………………………………………………………………………………………..14 Analytical Problems……………………………………………………………………………………………15 AP Multiple Choice Practice…………………………………………………………………………..16-17 AP Free Response Practice…………………………………………………………………………………19 2 1 Introduction: Try finding the following integral: ∫0 π π₯ 2 Using basic integration techniques, there truly is no way to analytically find an area. Yet, we know many ways to approximate the answer including a tangent line approximation, Riemann Sum, and also Euler’s Method. Yet there is one other method that was found by Brook Taylor and Colin Maclaurin within the 18th century. Taylor created the idea that all functions can be represented by polynomials that could be written as a series to infinity. As the series gradually approaches infinity, it begins to act more and more like the original function. The great part is since this series is a polynomial; it can easily be integrated and often converges to a certain number! Think of a pizza. Imagine you’re working in a pizza shop and get a pizza order with an infinite amount of toppings. As you put more toppings on the pizza, you gradually get closer and closer to completing the order. In our first example, we’re going to give you a pizza to make with an infinite amount of different toppings, and you’re going to try and fill the order as best you can, using a Taylor Polynomial. Examples 1 and 2: Conceptually understanding Taylor Polynomials, graphing them and writing a general term to find the Taylor Series. Goal: To write the 4th degree Taylor polynomial of a function and the general term of the Taylor Series, while developing a conceptual understanding of the series in its relation to the Consider the following function π(π₯) = π π₯ : This is our pizza order, and we want to write a polynomial, or Taylor Polynomial, that would allow us to complete it. First, we’ll need to pick our first topping, and we want it to meet the order at exactly one point so our approximation can be tangent to our original curve. To make it easy, lets pick our point to be x = 0. This number is nice since: π(0) = 1. From now on this point will be called our center, and we’ll say the Taylor polynomial is centered at 0, or c = 0. Here’s our graph, the red will represent the functionπ(π₯) = π π₯ , and the blue our polynomial, which is thus far π0 (π₯) = 1 3 Now, our pizza’s not finished. We’ve reached one topping, and our polynomial is centered so that it shares one point with the function, but we need to be able to get a better approximate. So, to do so we want to add another “degree” to our polynomial, now creating a first degree Taylor Polynomial. Because the zero-degree Taylor was a horizontal line, the next degree is going to have a slope, and essentially going to be a tangent line to the curve at the point (0,1) since our center is at c=0. To write this, we know that: π ′ (π₯) = π π₯ π ′ (0) = π 0 = 1 Our first degree Taylor polynomial is now going to read: π1 (π₯) = 1 + (π₯ − 0) π1 (π₯) = 1 + π₯ The graph of our curve and the polynomial is pictured to the right. Now we have a curve and a tangent line. As you can see we’re getting a more interesting pizza. Let’s add another degree, which will make our polynomial a quadratic. You can think of this as a tangent “parabola” to the curve. To make a quadratic polynomial, we have to add concavity, and use the second derivative of our function. π ′ ′(π₯) = π π₯ π′′(0) = π 0 = 1 The second degree Taylor polynomial will look like this: (π₯ − 0)2 π2 (π₯) = 1 + (π₯ − 0) + 2! π2 (π₯) = 1 + π₯ + π₯2 2! 4 If we continue this same approach using for the third degree, we’re going to get a polynomial that looks like this: π ′ ′′(π₯) = π π₯ π ′′′ (0) = π 0 = 1 (π₯ − 0)2 (π₯ − 0)3 π3 (π₯) = 1 + (π₯ − 0) + + 2! 3! π3 (π₯) = 1 + π₯ + π₯2 π₯3 + 2! 3! As you can see, we now have a tangent cubic function. Slowly but surely we’re developing a polynomial that is gradually going to fit the actual function, π(π₯) = π π₯ . For more proof, here’s the fourth degree polynomial: π (4) (π₯) = π π₯ π (4) (0) = π 0 = 1 (π₯ − 0)2 (π₯ − 0)3 (π₯ − 0)4 π4 (π₯) = 1 + (π₯ − 0) + + + 2! 3! 4! π4 (π₯) = 1 + π₯ + π₯2 π₯3 π₯4 + + 2! 3! 4! This is even closer to our actual function. Yum ο 5 If, hypothetically, we could take our polynomial to an infinite amount of degrees, we would be able to fill our pizza order and our polynomial would truly be the actual function:π(π₯) = π π₯ . But, instead of writing out every single term of our polynomial, we can write a term to nth degree of the Taylor Polynomial for this function, which we call a general term. π(π₯) = π π₯ = 1 + π₯ + π₯2 π₯3 π₯4 π₯π + + +β― 2! 3! 4! π! Represented by sigma notation, we have ∞ π₯π π(π₯) = π = ∑ π! π₯ π=0 This is the Taylor Series for the function, π(π₯) = π π₯ . The values it converges to for a certain “x” are the same you would find by plugging this “x” into the function. It successfully completes a pizza order, as all the toppings have been added. There’s one more phenomenon we have to look at before we move on, which is the factorial. Why do we add a factorial as we had degrees to our Taylor Polynomial? Well, look what happens when we take the derivative of the function π(π₯) = π π₯ . Derivative: π ′ (π₯) = π π₯ = 1 + 2π₯ 2! + 3π₯ 2 3! Simplified, this is just π ′(π₯) = 1 + π₯ + + π₯2 2! 4π₯ 3 4! + +β― π₯3 3! ππ₯ π−1 π! π₯π + β― π! = π π₯ Therefore, the factorials allow us to account for the exponent attached to each term of the polynomial, so that the derivatives and integrals of our polynomial hold true for the actual function. _______________ Any Taylor Series that is centered at c=0 (like above), is called a Maclaurin Series. This is simply just a special case scenario for a Taylor Series. Some common Maclaurin Series are: ∞ π₯2 π₯3 π₯4 π₯π π₯π π =1+π₯+ + + +β― =∑ 2! 3! 4! π! π! π₯ π=0 ∞ (−1)π π₯ 2π π₯2 π₯4 π₯6 π₯8 (−1)π π₯ 2π πππ (π₯) = 1 − + − + + β― =∑ (2π)! (2π)! 2! 4! 6! 8! π=0 6 ∞ π₯3 π₯5 π₯7 π₯9 (−1)π π₯ (2π+1) (−1)π π₯ (2π+1) π ππ(π₯) = π₯ − + − + + β― =∑ 3! 5! 7! 9! (2π + 1)! (2π + 1)! π=0 ∞ π₯2 π₯3 π₯4 π₯π (−1)π+1 π₯ π ππ(π₯ + 1) = π₯ − + − + β― =∑ 2 3 4 π π π=1 These Maclaurin Polynomials are used many times on the AP Exam, and it is recommended that you memorize them. Example 3: Analytical Problem Here’s a basic analytical problem that utilizes a bank to find a Taylor and Maclaurin Series. It’s an “easier” way to make a pizza, and is less conceptual than the method in Examples 1 and 2. Find the 3rd degree Taylor polynomial for the function π(π₯) = ln π₯ centered at c=2 The general term to develop any Taylor Series: ∞ π (π) (π ) ∑ (π₯ − π)π π! π=0 That general term is your plain pizza. But what good is a plain pizza if your customer wants one with pepperoni, onions, and hot peppers? How do we use the general form of the Taylor Series to generate a Taylor Polynomial that fits a more specific function? Well, ∞ π(π) (π) π(π) π′ (π) π′ ′(π) π′′′ (π) (π − π)π + ∑ (π − π)π = (π − π)π + (π − π)π + (π − π)π π! π! π! π! π! π=π …and on and on. 7 How to solve: So in order to TAYLOR our “pizza” to the customer’s order we need to gather the toppings they requested. The first step in writing a Taylor polynomial is to create a “bank” of the derivatives of the original function and their values at the center I in order to “fill out” our general form. Since the problem gives us that c=2, we know this is a Taylor polynomial and not one of our basic Maclaurin Polynomials. Here’s our bank: π (π) (π₯) π (π) (π) n=0 ln π₯ ln 2 n=1 1 π₯ 1 2 n=2 −1 π₯2 2 π₯3 −1 4 1 4 n=3 Now we just have to add the toppings and throw our pizza in the oven! (The pizza metaphor is getting a little painful, I know.) Here’s our general form for a third degree Taylor Polynomial: π(π) 0! (π₯ − π)0 + π ′ (π) 1! (π₯ − π)1 + π ′′ (π) 2! (π₯ − π)2 + π ′′′ (π) 3! (π₯ − π)3 So now we need we go back to our plain pizza and add the toppings we’ve just gathered. Use the values in the bank to plug in for f(c), f’(c), f’’(c), and f’’’(c) in your general form and add in your n!’s and c’s. ln 2 0! (π₯ − 2)0 + 1⁄ 2 (π₯ 1! − 2)1 + −1⁄ 4 (π₯ 2! − 2)2 + 1⁄ 4 (π₯ 3! − 2)3 Notice: this is the THIRD degree and we took the derivative THREE times. 8 Finally, we can simplify our polynomial and get our final answer: (π₯ − 2) (π₯ − 2)2 (π₯ − 2)3 ln 2 + − + 2 8 24 _______________________ So I think we’ve mastered the basics of making these pizzas (yup, the analogy’s still going). Let’s get a little more complicated. Example 4: AP Level Multiple Choice Problem π π The sixth degree Taylor Polynomial centered at π₯ = 2 of π(π₯) = sin(π₯ − 2 ) is (A) π₯3 π₯− (B) 1 − 6 π₯2 2 π₯5 + 120 π₯4 π 2 2 π (π₯− ) 2 2 π π 3 (π₯− ) 2 2 6 (C) (π₯ − ) + (D) (π₯ − ) − (E) 1 − π₯6 + 24 − 720 π 2 (π₯− ) 2 2 + + + π 4 (π₯− ) 2 24 π 4 2 (π₯− ) 24 + π 6 2 (π₯− ) 720 π 5 (π₯− ) 2 120 − π 6 2 (π₯− ) 720 How to solve: Now, this is actually an easy problem in disguise. Even though the problem asks for a π π Taylor Polynomial centered at π₯ = 2 , because the function is shifted over to π₯ = 2 as well, our polynomial will essentially act like a Maclaurin polynomial for sin π₯. The fact that the question asks for the sixth degree polynomial is moot because the 6th degree of the Maclaurin of sin π₯ is zero so the correct answer only goes up to the 5th degree. However, all of the (π₯ − π)π will π π become (π₯ − 2 ) . This makes options (A) and (B) incorrect, as they are just the Maclaurin of sin π₯ and cos π₯, respectively. Option (E) is a polynomial for cos π₯. Finally, (C) is totally incorrect, as it behaves unlike any common Maclaurin polynomial. Therefore, the correct answer is choice (D). __________________ Now for the big pie. Here’s an example of a tough AP Level Free Response problem. This would most likely be at the end of the free response section, and the last section of a problem. 9 Example 5: AP Level Free Response 2 The function π(π₯) = π 2π₯ and π(π₯) = π(π₯)−1 2π₯ 2 for all real numbers except x=0. π(π₯) = 0 when π₯ = 0 If the function g(x) is continues and differentiable for all real numbers, does g(x) have a relative minimum, maximum or neither at x=0? Justify your answer. ________________ How to solve: We need to think. A relative minimum or maximum on the graph of g(x) can only occur where the derivative of g(x)=0, and the derivative changes signs. Because the AP says to justify the answer, we need to show this work. First, let’s look at the function g(x). It would be very hard to differentiate this, since we would have to use a quotient rule, and our function f(x) isn’t exactly an easy pizza. But, the function f(x) is like a pretty common pizza we know, just a little more, “flavorful.” The function f(x) is very like π π₯ . Now, why is this important? Well, this problem is actually a Maclaurin Series question in disguise; it’s a very tricky pizza that’s actually not too hard to make. To notice this, think of the Maclaurin series for π π₯ which follows: ∞ π₯2 π₯3 π₯4 π₯π π₯π π₯ π =1+π₯+ + + +β― =∑ 2! 3! 4! π! π! π=0 Now, to make the series for f(x), all we have to do is plug in 2π₯ 2 to our series: 2 π 2π₯ = 1 + 2π₯ 2 + 4π₯ 4 8π₯ 6 16π₯ 8 (2π₯)2π + + +β― 2! 3! 4! π! Now look at this series, and the function g(x). If we plus this series into g(x), something very cool happens. Our series becomes a lot simpler, and many terms cancel out. The AP tends to create its problems so that they work out so nicely, and this pizza is slowly getting more delicious. Also, most likely this would be the end of a free response question, and there is a great chance that the previous questions in the problems would have us develop the Maclaurin series for π(π₯) and π(π₯) already. Therefore this phenomenon would be a little more obvious to us. 10 (2π₯)2π 4π₯ 4 8π₯ 6 16π₯ 8 2 [1 + 2π₯ + + + + β― π(π₯) − 1 2! 3! 4! π! ] − 1 π(π₯) = = 2π₯ 2 2π₯ 2 2π₯ 2 4π₯ 4 8π₯ 6 2π π₯ 2π =1+ + + +β― 2! 3! 4! π! Now this series is pretty easy to work with and differentiate. Let’s do that: π ′ (π₯) 4π₯ 16π₯ 3 48π₯ 5 = + + +β― 2! 3! 4! …and plug in 0 to see whether a relative maximum or minimum may exist at this point. π′ (0) = 0 since g(x) = 0 for x = 0 Now we know that a relative maximum or minimum may exist at this point, but we have to figure out what type of extrema exists here, if one does truly exist. Normally, we could use a first derivative test, and show the derivative changes signs at x=0, but this would be quite complicated because we would have to figure out the convergence of our derivative is when x does not equal 0. Well, instead, we can try a second derivative test. Look what happens when we find our second derivative: π′ ′(π₯) = 4 48π₯ 2 (48 ∗ 5)π₯ 4 + + +β― 2! 3! 4! And plug in x=0 π′′ (π₯) = 4 48(0)2 (48 ∗ 5)(0)4 4 + + +β―= =π 2! 3! 4! 2! Again, all terms going to infinity will have an “x” in it, and they will cancel out to 0. We know now that the second derivative is positive at x=0, and the first derivative is 0 at this point. This provides us enough information to make the claim that there is a relative minimum at x=0. Here’s a sample justification for this problem: “Since g’(x)=0 at x=0, and g’’(x) is positive at x=0, we know that the graph of g(x) is concave up at x=0, and there is a relative minimum at this point.” __________________ So..finally let’s look at our original problem, and see how easy it is to complete our order! 11 2 Example 6: Write the first three nonzero terms for the Taylor Polynomial π(π₯) = π π₯ centered 1 2 at c=0, and use it to approximate ∫0 π π₯ . ______________ How to solve: 2 First off, we need to write the first three nonzero terms of π(π₯) = π π₯ . Let’s make our pizza order simpler. We know that: ∞ π₯2 π₯3 π₯4 π₯π π₯π π₯ π =1+π₯+ + + +β― =∑ 2! 3! 4! π! π! π=0 So therefore the first three nonzero terms are: π π₯2 π₯4 ≈ 1+π₯ + 2! 2 Now, we integrate this function and get ∫π π₯2 π₯3 π₯5 ≈π₯+ + 3 5 ∗ 2! Pretty simple. All that’s left is finding the definite integral. 1 2 ∫ π π₯ ≈ (1 + 0 13 15 03 05 π π + ) − (0 + + )=π+ + 3 5 ∗ 2! 3 5 ∗ 2! π ππ Ta duh! We’ve completed our order and approximated our integral! Remember, you do not have to simplify your answer (counter to what most non-cool textbooks think). Just plug in your numbers and you’re done. The most important thing that the AP will want to see is the Calculus part of the problem, and we have already shown that. ___________________ Simple enough! Hopefully by now you can see how easy it is to write a function as a Taylor series, and also how useful this series can be. Just like working in a pizza shop, we can take a long and confusing order, and break it down into simple toppings. So for now… Bon appetite! ο 12 Pizza History Now…let’s take some time to learn about one of the famous chefs who contributed to making one of our wonderful pizzas…Colin Maclaurin! Colin Maclaurin Many believe that Colin Maclaurin should not be accredited with the series that holds his name, seeing it is truly just a special case for a Taylor Series for c=0. But, in reality, it is not Maclaurin’s Series that gives him fame, but the applications he found with the series that Taylor did not. Colin Maclaurin lived in Scotland for most of his childhood, and then attended the University of Glasgow. He was appointed deputy to the mathematical professor at Edinburgh upon the recommendation by Isaac Newton. Newton eventually appointed Maclaurin as the professor, after Maclaurin bought an overly obscene powdered wig. As the wig aged, became grey and whiskers began to appear on Macluarin’s face, he decided to write a book to show the knowledge he had developed. This book, called Treatise of Fluxions, became Maclaurin’s most famous work. In it, he showed how one can use a Taylor Series to find minimums, maximums, take derivatives and approximate integrals. Of course, the main focus became the special case Taylor Series where c=0, the series which holds Maclaurin’s name today. He was not a pompous man (though his wig was overly pompous), and did accredit Taylor with all the work. Yet, we deemed Maclaurin important enough to have his own series. Other work of Maclaurin included the integral test to find whether an infinite series converges, and he applied Calculus to figure out properties of ellipses and how they act in nature. Once Newton passed away, Colin Maclaurin and the animal that sat on top of his head helped provide much of the work that would allow Calculus to flourish. He became ill in 1745, passing away in Edinburgh the following year. His wig was never found. Also, it would be nice to know why we need Taylor and Maclaurin Series, because they’re not just really really fun abstract math problems. They can truly be enjoyable pizzas because of how useful they are! 13 Applications Taylor and Maclaurin Series have many interesting applications, and they usually “revolve” around cyclical functions. First of all, functions like cosx, sinx and π π₯ only have rational answers for specific numbers. Therefore, we use calculators to find the definite answers to lets say, π 2 . But, in reality calculators and computer cannot even find the true answer either, because these functions produce irrational numbers. So, what calculators actually do is use a Taylor polynomial to a very high degree to approximate answers for these functions. Therefore, Taylor approximations are used all around us. Also, many other topics in physics make use of these cyclical functions. Pendulums, sound waves and anything that oscillates produce equations with sinx and cosx. Many times these functions cannot be differentiated or integrated, and Taylor and Maclaurin approximations are the only methods to completing certain problems. Taylor and Maclaurin Series can be used to approximate any function! So, they are truly versatile pizzas, and can be eaten for breakfast, lunch, or dinner. I LOVE PIZZA!! 14 Bibliography http://www-history.mcs.st-and.ac.uk/Biographies/Maclaurin.html http://mathworld.wolfram.com/TaylorSeries.html http://www.ugrad.math.ubc.ca/coursedoc/math101/notes/series/powers.html 15 Analytical Problems For questions 1 & 2, complete the derivative “bank” for the given function. 8. n= 3 , c = 3π 4 1 1. π(π₯) = sin π₯ , π = 2 π(π₯) π ′ (π₯) π ′′ (π₯) π (3) (π₯) π (4) (π₯) π₯ Using the general form of the Maclaurin polynomials as a starting point, find the nth degree polynomial for the given function. π 9. π(π₯) = − sin π₯ , π = 5 10. π(π₯) = π 3π₯ , π = 3 2. π(π₯) = ln π₯ , π = 1 π(π₯) π ′ (π₯) π ′′ (π₯) π (3) (π₯) π (4) (π₯) 11. π(π₯) = sin ππ₯ , π = 5 12. π(π₯) = cos 4π₯ , π = 4 π₯ π Use a derivative bank to find the Maclaurin polynomial of the nth degree for the given function. Sketch On the same graph, sketch the graph for (a) the given function and (b) the Taylor approximations of the function for n = 1, 2, and 3 centered at the given c value. You may use a graphing calculator to aid your sketches but be sure to note points of intersection. 3. (π₯) = π π₯ , n = 4 4. (π₯) = cos π₯ , n = 3 5. π(π₯) = cos 2π₯ , n = 6 For questions 6-8, find the Taylor Polynomial of the nth degree centered at c for π(π₯) = sin π₯ + cos π₯ 6. n = 3 , c = 0 π 7. n = 2 , c = 2 π 13. π(π₯) = sin 2 π₯ , π = 0 14. π(π₯) = 2 ln π₯ 3 , π = 2 15. Writing In your own words, explain how Taylor series work to approximate the values of a function. Be sure to include a description of the visual justification. 16 AP Multiple Choice Questions- Taylor Series 1. What is the 6th degree Maclaurin Polynomial for π(π₯) = sin(2π₯)? a. π₯ − π₯3 + 3! 5! 8π₯ 3 32π₯ 5 b. 2π₯ − c. 2π₯ − 3! 8π₯ 3 3! d. 1 + π₯ + e. 1 − π₯5 + 5! 32π₯ 5 + π₯2 5! π₯6 − 7! + 2! 6! 4π₯ 2 16π₯ 4 + 2! 128π₯ 7 − 4! 64π₯ 6 6! 2. If π(2) = 1, π ′ (2) = 0, π ′′ (2) = 3, π ′′′ (2) = 0 and π (4) (2) = 5, what is the fourth degree Taylor Polynomial for f(x) where c=2? a. 1 + b. 3(π₯−2)2 2! (π₯−2)4 4! c. 1 + d. 1 − e. 1 + + 3π₯ 2 + 5(π₯−2)4 (π₯−2)6 + 6! 5π₯ 4 4! (π₯−2)8 + 8! 2! 4! 2 3(π₯−2) 5(π₯−2)4 + 2! 3(π₯−2)4 2! + 4! 5(π₯−2)8 4! 17 3. The Taylor Polynomial of degree 80 for a function with derivatives of all orders about c=2 is given by: (π₯ − 2)4 − (π₯ − 2)8 (π₯ − 2)12 (π₯ − 2)16 (π₯ − 2)4π (π₯ − 2)80 + − + β― (−1)π+1 + β―− 2! 3! 4! π! 20! What is π (40) (2)? a. b. c. d. e. 40! 20! 40! 10! −40! 10! −40! 20! −20! 40! 4. What is the Taylor Series of ln(π₯ + 1) centered at c=0? a. b. c. d. e. π₯π π! π π₯ π+1 ∑∞ π=1(−1) π (π₯+1)π ∞ π ∑π=1(−1) π π₯π ∞ π+1 ∑π=0(−1) π π₯π ∞ π ∑π=1(−1) π π+1 ∑∞ π=1(−1) 5. Using the fifth degree Maclaurin Polynomial for cos(π₯), approximate ∫ cos(π₯ 2 ). π₯3 π₯5 a. π₯ − 3∗2! + 5∗2! + πΆ b. π₯− c. d. e. π₯5 π₯6 + +πΆ 3! 5! π₯2 π₯4 1− + +πΆ 2! 4! π₯5 π₯9 π₯− + +πΆ 5∗2! 9∗2! 3 7 11 π₯ π₯ π₯ − 7∗3! + 11∗5! + πΆ 3 18 AP Calculus BC Taylor Series Free Response Let the function π(π₯) = sin(π₯ 2 ) + cos(π₯ 2 ) where F(0)=1 a. Find the sixth degree Maclaurin polynomial for both sin(π₯ 2 ) and cos(π₯ 2 ) b. Using your answers from part (a), find the sixth degree Maclaurin polynomial for f(x) as well as the general term for the series π₯ c. Using your sixth degree Taylor Polynomial from part (b), approximate ∫0 π(π‘)ππ‘. Use 1 that answer to approximate ∫0 π(π‘)ππ‘