Study Guide

The Fundamental Theorem of Calculus - In the Real World

In the Real World

We could try to point out different careers in which you need to use what we've just been doing, but we're not going to bother. Let the textbooks do that.

A large part of the practicality of this unit lies in the way it stretches your brain. Thinking backwards to find antiderivatives and understanding the FTC are exercises that will help your general thinking and problem-solving skills. Also, if you understand this stuff first then you can help that cute boy/girl in your class who's having trouble. What could be more practical than that?

On a more immediate note, you're going to need these tools for the rest of calculus. When we get to density and probability, for example, a lot of questions will ask things like

"For what value of M is 

Practice now, save yourself headaches later!

  • I Like Abstract Stuff; Why Should I Care?

    Most of the functions we deal with in calculus are elementary functions. Intuitively, elementary functions are the ones you can write a nice formula for; the ones you know what to do with.

    They're the power functions, logarithmic functions, exponential functions, trigonometric functions and their inverses, and all the functions you can build out of these by adding, multiplying, dividing, taking nth roots, and composing functions.

    It might seem like we've just described every function there is. What could possibly be left? One example of a non-elementary function is

    This is a function, but there's no way to write a nice formula for it using any combination of power, log, exponential, and trig functions.

    Remember that there are more irrational than rational numbers? There are also more non-elementary functions than there are elementary functions. We don't encounter them as often in math classes because they're harder to think about. However, if you put all the functions that take real numbers as input and give real numbers as output up on a wall and threw a dart at them, you would probably hit a non-elementary function.

  • How to Solve a Math Problem

    There are three steps to solving a math problem.

    1) Figure out what the problem is asking.

    2) Solve the problem.

    3) Check the answer.

    Sample Problem

    Lou paints at a rate of (140 – kt) square feet per hour, where t is the number of hours since he started painting and k is a positive constant accounting for the fact that Lou slows down as he gets tired. If Lou takes 6 hours to paint 500 square feet, how long does it take him to paint the last 100 square feet?

    Answer.

    1) Figure out what the problem is asking.

    The problem asks how long Lou takes to paint the last 100 square feet. We know he finishes painting when t = 6. We need to know when he started painting those last 100 square feet—that is, for what value of T is

    This is an equation involving an integral. Unfortunately, it has letters both in the integrand and in the limits of integration:

    If it weren't for that pesky "k" in the integrand we could work out the integral and solve the equation. This means we need to figure out what k is. Thankfully, the problem tells us enough to do that. We know Lou takes 6 hours to paint 500 square feet, so

    We can solve this equation for k, then go back to the first equation and find T.

    2) Solve the problem.

    First we find k:

    We need to use k for the next part, so we keep the exact answer

    instead of rounding.

    Now that we know k, we can solve the equation that will tell us the time at which Lou started painting the last 100 square feet:

    Rearranging, we get a (horrible) quadratic equation:

    Let's multiply through by 9 to make it slightly less horrible:

    0 = 85T2 – 1260T + 3600.

    Still horrible, but at least now we can apply the quadratic formula without worrying about having fractional coefficients.

    The possible answers are

    and

    Since Lou finished his painting in 6 hours, he couldn't have started painting his last 100 square feet after 10.959 hours. This means the other number must be the one we want. It's reassuring that 3.865 is in between 0 and 6. Rounding to the nearest minute, we have

    T ≈ 3.865(60) = 232 minutes,

    or 3 hours and 52 minutes.

    To finish, we need to answer the question that was actually asked, which was "how long" it takes Lou to paint those last 100 square feet. He started at 3 hours and 52 minutes, and finished at 6 hours, so it took him 2 hours and 8 minutes to paint the last 100 square feet.

    3) Check the answer.

    We've already done some checking along the lines of "does this answer make sense?" We know Lou painted for 6 hours and we found that he started painting the last 100 square feet after more than 0 but fewer than 6 hours, which is good.

    Since this problem mostly consisted of finding values for k and T that made certain integrals come out to specific values, one way to check our answers is to take the values we found for k and T, stick the integrals into a calculator, and make sure they come out as they're supposed to.

    We know the integral

    is supposed to come out to 500, and we found . Stick

    this value in for k, and put the resulting integral in your calculator:

    You should get 500, which means we found the correct value of k.

    Next, we need to see if we found the right value of T to make

    come out to 100. We said Lou started painting his last 100 square feet at 232 minutes, or  hours. Since we rounded to the nearest minute this isn't an exact start time, which means when we plug in  for T we expect the integral to come out close to but not exactly equal to 100. Stick the following integral into your calculator:

    We get about 99.87. You might get a slightly different answer, but it should still be close enough to 100 to convince you that we found the correct value of T.

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