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Topic: Historyu of the integral of secant
Replies: 13   Last Post: Mar 3, 2009 3:11 PM

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Stephen J. Herschkorn

Posts: 2,297
Registered: 1/29/05
Re: History of the integral of secant
Posted: Mar 1, 2009 6:38 PM
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Stephen J. Herschkorn wrote:

> What is the history of the trick to find the indefinite integral of
> the secant function? Who first used it, and when?



Thanks to all for the responses to my query. From these, I summarize
that Mercator (whose work precedes that of Newton and Leibniz by about
two centuries) showed that integral(x=0..t, sec x) = ln |tan t/2 +
pi/4|, though not in that notation. I suspect that, after the
development of calculus, if one notices that ln |tan t/2 + pi/4| = ln
|sec t + tan t|, one can then come up a posteriori with the trick of
converting sec x to sec x (sec x + tan x) / (sec x + tan x ).

As I noted in another post, this all comes up because I was presenting
the integral of secant, via the aforementioned trick, in a
second-semseter calculus class the other day. (I have been teaching
calculus for only two years now, and this is the first time this topic
has come up in the syllabus.) The students very naturally asked, "How
did you know to multiply by sec x + tan x?" My response was, well,
that's just a trick someone came up with. I wouldn't expect the
students to come up with it on their own.

Rick Decker, in another post in this thread, posted another derivation.
Rewriting it a bit,

sec x = 1 / cos x = cos x / cos^2 x = cos x / (1 - sin^2 x).

Now use the substitution u = sin x and apply partial fractions to get

1/2 [ln(1 - sin x) + ln(1 + sin x)]

as an antiderivative. Now use the properties of logarithms and the
trigonometric functions to show that this equals ln |sec x + tan x|.

This derivation seems more "natural" to me: Asked to find the
antiderivative of the secant, I see nothing unusual in coming up with
it. Once one sees the final result, one can then come up with the
efficient trick. I wouldn't be surprised if this how it all went down.

--
Stephen J. Herschkorn sjherschko@netscape.net
Math Tutor on the Internet and in Central New Jersey




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