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Topic: [ap-calculus] Derivatives of trig functions
Replies: 2   Last Post: Aug 29, 2012 10:52 PM

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Alan Lipp

Posts: 1,091
Registered: 12/6/04
RE: [ap-calculus] Derivatives of trig functions
Posted: Aug 29, 2012 6:56 PM
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Good point, Dan, and a very nice addition to the exercise.

Alan

-----Original Message-----
From: Teague, Dan [mailto:teague@ncssm.edu]
Sent: Wednesday, August 29, 2012 6:46 PM
To: Alan Lipp; AP Calculus
Subject: RE: [ap-calculus] Derivatives of trig functions

Steve,

I agree with Alan here, except I prefer to use y2 =
(y1(x+.0001)-y1(x))/.0001 instead of the Nderiv to reinforce the definition.
If y1 = sin(x) and your students don't tell you that y2 is the graph of the
cosine function, then... Well, I guess I don't believe it is possible for
them not to tell you y2 is the cosine function.

Dan

Daniel J. Teague
NC School of Science and Mathematics
1219 Broad Street
Durham, NC 27705
teague@ncssm.edu
________________________________________
From: Alan Lipp [alipp@crocker.com]
Sent: Wednesday, August 29, 2012 1:17 PM
To: AP Calculus
Subject: RE: [ap-calculus] Derivatives of trig functions

Hi Steve,

I do derive these formulas, but only after my students have guessed them.
There are lots of apps available online now, but I've used a simple
application of the TI series of calculators. First graph y1 = f(x) . . .
your choice, and then y2 = Nderiv(y1, x, x). This graphs both your
starting function and a numerical approximation to the derivative. For the
trig functions you can stop here since virtually everyone will guess the
correct derivatives. I then have them graph y3 = their guess. If they are
correct nothing seems to happen since y3 and y2 overlap. I choose to be
skeptical and ask how seeing nothing can prove they are correct.

The kicker is to unhighlight y1, y2, and y3 and graph y4 = y2/y3. If y3 is
correct then y4 will appear to be the line y = 1, supporting their guess.

The real advantage of this method is that you can now change y1 to any
function at all, graph y2, have them guess y3, and use y4 to confirm or
refute their guess. Even when they are wrong y4 can yield important
information. For example, graph y1 = x^3. The graph of y2 is a parabola
and most students will guess y3 = x^2 (I am assuming they have not yet
learned the power rule.) Graph y4 and you will see a horizontal line. A
trace, however, shows that y4 = 3, and a little algebra shows that if y2/x^2
= 3 then (x^3)' = 3x^2. My kids manage to guess most of the standard
formulas long before I have a chance to prove them so they get a week or
more of derivative practice before they see the derivation. I am always
surprised that they manage to guess the derivatives of tan(x) and a^x. I
like this investigation (which usually takes one class and a follow-up
homework) both because it gives my students the opportunity to be creative,
and also because they then have an investment in the correct answers which
provides motivation for the derivations that follow.

Alan


Alan Lipp
Williston Northampton School







-----Original Message-----
From: Steven Jonak [mailto:steven.jonak@kirkwoodschools.org]
Sent: Wednesday, August 29, 2012 8:12 AM
To: AP Calculus
Subject: [ap-calculus] Derivatives of trig functions

I'm planning my lesson on the derivatives of the trig functions and am
looking for alternate ideas. I normally derive the derivative of sine and
cosine using the definition of derivative and then the quotient rule for the
rest. I'd like to do something a little different. Any ideas? Does
everyone use the definition of derivative to get at sine and cosine
derivatives?
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Course related websites:
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http://apcentral.collegeboard.com/calculusbc
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