Tag Archives: The “Why …” Series

Why Review?

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The reason you review is TO MAKE MISTAKES!

When you’re reviewing for the AP Calculus exams your goal is to make mistakes. Why make mistakes? Easy: to find out what you’re doing wrong so you can fix it. And to find out what you don’t know so you can learn it.  

Your teacher will assign free-response questions, FRQs, from real AP Calculus exams from past years. Give yourself about 15 minutes and try to answer the question. (Fifteen minutes is about the time you have for each FRQ on the exam.) After fifteen minutes, stop. Check your work.

The questions, answers, solutions, and most importantly the scoring guidelines for FRQs are all online here for AB, and here for BC. Each FRQ is worth nine points. The scoring guidelines will show you what must be on your paper to earn each point.

Now you can copy the absolutely perfect answer for your FRQs and hand it in to your teacher. This won’t impress or fool your teacher because he or she has the guidelines too. More importantly this won’t help you. When reviewing mistakes are good. Study your mistakes and learn from them.

  • If you made a simple arithmetic or algebra mistake, learn to be more careful. One very common mistake is simplifying your answer incorrectly. Remember, you do not have to simplify numerical or algebraic answers. If you write “ 1 + 1 “ and the correct answer is 2, the “ 1 + 1 ” earns the point. But if you simplified it to 3, you lose the point you already earned. (The standards show simplified answers, so the readers will know what they are for (foolish?) students who chose to simplify.) simplifying also wastes time.
  • If you’re unsure how to write justifications, explanations, and other written answers, use the scoring guidelines as samples or templates. Learn to say what you need to say. Don’t say too much. You will not earn full credit for the correct answer and correct work if the question asks for a justification, and you don’t write one.
  • If you really don’t know how to answer a question you’ve made an important mistake. This is the thing you need to work on until you understand the concept or method. This requires more than just reading the solution on the guideline. Go back to your notes, ask your friends, ask your teacher, find out what you’re missing and learn it. Look at similar questions from other exams.
  •  For multiple-choice questions only the answers are available. Nevertheless, be sure you understand your mistakes.

There are 7 types of questions on the AB exam and an additional 3 on the BC exam. These are not the same as the ten units you’ve been studying, because AP exam questions often have parts from more than one unit. On March 5, I will post links to all the types. The discussion of each type will include a list of what you should know and be able to do for each type along with other hints.           

Now, when you actually take the AP Calculus exam your goal changes. Here you want to earn all the points you can. If you run across something you know you don’t know on the exam, leave it. Go onto something you do know. Don’t waste your time on something you’re not sure of. You can always come back if you have time.

Missteaks our heplfull.

Why Convergence Tests?

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A large amount of time in Unit 10 is devoted to convergence tests. These tests tell you under what conditions a series will converge, when the infinite sum will approach a finite number.

The tests are really theorems. As with all theorems, you should learn and understand the hypotheses. This summery of the convergence tests lists the hypotheses of the tests that you are expected to know for the AP Calculus BC exam. The conclusion (at the top) is always that the series will converge or will not converge. You will likely spend a day or two on each test, learning how and when to use it. Use the summary to help you.

Some series have both addition and subtraction signs between the terms (often alternating). A series is said to be absolutely convergent or to converge absolutely if the series of absolute values of its terms converges. In effect, this means you may determine convergence by ignoring the minus signs. If a series converges absolutely, then it converges. This is an important way that many alternating series and series with some minus signs may be tested for convergence. If a series does not converge absolutely, it may still converge. In this case the series are said to be conditionally convergent.

Your goals is to learn which test to use and when to use it.  The short answer is that you may use whichever test works. There is often more than one. Here are two blog posts discussing this. Read these after you’ve learned the convergence tests (but before your teacher’s test). The first post shows how different tests may be used on the same series. The second post gives hints on which test to try first. The key is the standard advice: Practice. Practice. Practice.

Course and Exam Description Unit 10, Sections 10.2 to 10.9. This is a BC only topic.

Why Power Series?

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The polynomial function \displaystyle f\left( x \right)=x-\tfrac{1}{6}{{x}^{3}}+\tfrac{1}{{120}}{{x}^{5}}  approximates the value of \displaystyle \sin \left( {\tfrac{\pi }{6}} \right)correct to 5 decimal places:

\displaystyle f\left( {\tfrac{\pi }{6}} \right)\approx 0.500002

\displaystyle \sin \left( {\tfrac{\pi }{6}} \right)=0.5

This is not a fluke!

The graph of f(x) is in blue, the sin(x) in red. Note how close the two graphs are in the interval [-2, 2]

Now, approximating the value of a sine function is easier with a calculator. But sines are not the only functions in Math World.

In the Unit 10 you will learn how to write special polynomial functions, called Taylor and Maclaurin polynomials, to approximate any differentiable function you want to as many decimal places as you need. You already know a lot about polynomials. They are easy to understand, evaluate, and graph. The concept of using a polynomial to approximate much more complicated functions is very powerful.

You’ve already got a start on this! Recall that the local linear approximation of a function near x = a is \displaystyle f\left( x \right)\approx f\left( a \right)+{f}'\left( a \right)\left( {x-a} \right). This is a Taylor Polynomial. And it is the first two terms all the higher degree Taylor polynomial for f near x = a.

To fully understand these polynomials, there is a fair amount of preliminary stuff you need to understand. First you study sequences – functions whose domains are whole numbers. Next comes infinite series. A series is written by adding the terms of a sequence. (Sequences and series may have a finite or infinite number of terms. There is not much to say about finite series; infinite sequences and infinite series are where the action is.) oThe terms 0f some sequences and series are numbers. Other series have powers of an independent variable; these are called power series.   

Some power series approximate (converge to) the related function everywhere (i. e. for all Real numbers). Others provide a good approximation only on an interval of finite length. The intervals where the approximation is good is called the interval of convergence. Convergence tests – theorems really – help you determine if a series converges. These in tern help you find the interval of convergence. More on this in my next post.

Depending on your textbook and your teacher, you may study these topics in this order: sequences, convergence test, series, Taylor and Maclaurin polynimials for approximations, and power series. Others may change the order. The path may be different, but the destination will be the same.

Course and Exam Description Unit 10, Sections 10.1, 10.2, 10.11, 10.13, 10.14, 10.15. This is a BC only topic.  

Why Differential Equations?

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Differential equations are equations that include derivatives. Their solution is not a number, but rather a function which along with its derivative(s) satisfies the equation. That is, when the function and its derivative(s) are substituted into the differential equation the result is true (an identity). You may check your solution by substituting into the differential equation.

Differential equations are used in all areas of math, science, economics, engineering, and anywhere math is used. Derivatives model the change in something. Change is often easier to model (measure and write equations for) than the function that is changing. By solving the differential equation, you find the equation that describes the situation.

If it were only that easy. Differential equations are notoriously difficult to solve. In this, your first look at them, you will study the basics and only one of the many, many methods of solution. This is just to give you a hint of what differential equations are about.

Solution involves finding antiderivatives that include a constant of integration. The solution with an unevaluated constant is called the general solution. The solution could go through any point in the plane depending on the value of the constant of integration.  

To evaluate this constant, you must know a point on the solution function. This is called an initial condition, an initial point, or a boundary condition. Once the constant is evaluated, the result is called the particular solution.

A slope field is a technique for looking at all the solutions and seeing properties of the solutions. A slope field is a series of short segments regularly spaced over the plane that have the slope indicated by the differential equation. The segments are tangent to the solution curve through the points where they are drawn. You may start at any point (the initial condition point) and sketch an approximate solution by following the slope field segments. Doing so gives you an idea of a particular solution.

You will look at exponential functions as an example of an application of a differential equation.

BC students will also learn a numerical approximation technique called Euler’s Method. This is based on the linear approximation idea repeated several times. They will also look another model for the
Logistic equation.

Course and Exam Description Unit 7

Why Antiderivatives?

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Antiderivatives are needed to evaluate definite integrals.

The next thing to consider is how to find antiderivatives.

Each of the formulas you learned for finding a derivative may be reversed to find antiderivatives. For example, since \displaystyle \frac{d}{{dx}}\sin \left( x \right)=\cos \left( x \right), it follows that.\displaystyle \int{{\cos \left( x \right)dx}}=\sin \left( x \right)

I wish it were all that simple.

There are three concerns.

First, if the original function included a constant, this constant will disappear when you differentiate. Think about it: adding a constant translates the graph up or down but does not change the shape; the slope (derivative) remains the same.

This means that a function has an infinite number of antiderivatives. The good news is they are all the same except for the constant of integration.

The \displaystyle \cos \left( x \right)is the derivative of \displaystyle \sin \left( x \right)+3,\sin \left( x \right)-8,\pi +\sin \left( x \right) and all kinds of similar things.

To remind you of this you should write \displaystyle \int{{\cos \left( x \right)dx}}=\sin \left( x \right)+C where C is a constant, a number, called the constant of integration.

Next, very similar looking functions have very different antiderivatives found in very different ways. I won’t scare you with examples, you’ll see them soon enough.

Finally, there are many simple looking functions, that you can easily differentiate that do not have an antiderivative that is any function you’ve seen.

In the last parts of Unit 6, you will learn some methods integration. BC students will learn a few additional methods. You’ve only scratched the surface: there are many more, but these can wait until you get to university (or maybe Mathematica knows them – I wouldn’t be surprised).

As you learn these methods of integration you will have to decide when to use each. Learn which method is appropriate in each situation.

Course and Exam Description Unit 6.8 thru 6.14

Why the Fundamental Theorem of Calculus?

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The Fundamental Theorem of Calculus is, well, fundamental. It relates the derivative and the integral.

Writing a Riemann sum with all that fancy notation is tedious. To speed things up a special notation is used to replace it. The limit of the Riemann sum for a function on an interval [a, b] is written as its definite integral:

\displaystyle \underset{{\left| {\Delta x} \right|\to 0}}{\mathop{{\lim }}}\,\sum\limits_{{i=1}}^{\infty }{{f\left( {{{x}_{i}}} \right)\Delta x}} \displaystyle =\int_{a}^{b}{{f\left( x \right)dx}}

The \displaystyle f\left( x \right) (called the integrand) is the function with no fancy notation and the dx, called differential x replaces the \displaystyle \Delta x. The a and b, called the lower and upper limit of integration respectively, show you the interval the Riemann sum was formed on (which the Riemann sum does not).

Keep in mind that behind every definite integral is a Riemann sum. Therefore, all the properties of limits apply to definite integrals. They can be added and subtracted, a constant may be factored out, and so on.

The Fundamental Theorem of Calculus, the FTC, tells you how to evaluate a definite integral (and therefore its Riemann sum): Simply evaluate the function of which \displaystyle f\left( x \right) is the derivative at the endpoints of the interval and subtract.

To keep this in mind you can write the FTC like this considering the integrand as the derivative (of something):

\displaystyle \int_{a}^{b}{{{f}'\left( x \right)dx}}=f\left( b \right)-f\left( a \right).

For example, since  \displaystyle d\sin \left( x \right)=\cos \left( x \right)dx,

\displaystyle \int_{0}^{{\pi /2}}{{\cos \left( x \right)dx=\sin }}\left( {\tfrac{\pi }{2}} \right)-\sin \left( 0 \right)=1-0=1

That’s all there is to it!

But wait! There’s more! This reveals another important idea: Since derivatives are rates of change, the FTC says that the integral of a rate of change is the net amount of change over the interval. Also called the accumulated change.

Well, okay, there is the problem of finding the function whose derivative is the integrand which is not always easy. This function is called the antiderivative of the integrand; another name is the indefinite integral. (The notation for an antiderivative or indefinite integral is the same as for a definite integral without the limits of integration). The truth is that finding the antiderivative is not as straightforward as finding the derivative. We will tackle that soon.

Course and Exam Description Unit 6.3 thru 6.7

Why Optimization?

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Unit 5 ends with a return to a realistic context. To optimize something means to find the best way to do it. “Best” or “optimum” may mean the quickest, the cheapest, the most profitable, or the easiest way to do something.

For example, you may be asked to build a box of a given volume with the least, and therefore cheapest, amount of material. Thus, these are really problems where you need to find the maximum or minimum of the function that models the situation.  There are applications to engineering, finance, science, medicen, and economics among others.

The most difficult part of these problems is often writing the equation to be optimized; not the calculus involved. Once you have the model, finding the extreme value is easy.

The last part of this unit extends the ideas of this unit to implicit relations, those whose graph may not be a function. These too, increase, decrease, and have extreme values. The same techniques help you to find them.  

Course and Exam Description Unit 5 Sections 11 and 12

A note for teachers: You are not behind scheduel. Please remember that I am posing this series ahead, probably well ahead, of where you are. This is so that they will be here when you get here.