Wikipedia:Reference desk/Archives/Mathematics/2022 August 6

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August 6

Begging the question and L'Hôpital's rule

What is $\lim _{x\to 0}{\frac {\sin {x}}{x}}$? If one uses L'Hôpital's rule, then one would get $\lim _{x\to 0}{\frac {\cos {x}}{1}}=\cos {0}=1$. However, proving that the derivative of sine is cosine requires using the original limit. Could this be considered begging the question? GeoffreyT2000 (talk) 15:30, 6 August 2022 (UTC)

There are other ways of establishing the result, such as

${\frac {d}{dx}}\sin x={\frac {d}{dx}}\operatorname {Im} e^{ix}=\operatorname {Im} ({\frac {d}{dx}}e^{ix})=\operatorname {Im} (ie^{ix})=\operatorname {Re} e^{ix}=\cos x.$

Or, using the fact that the derivative of the cosine function is minus the sine, take the derivative of both sides of the equality $\cos ^{2}x+\sin ^{2}x=1$ and simplify.  --Lambiam 17:00, 6 August 2022 (UTC)

My general remark here would be that beginners tend to overestimate l'Hôpital's rule. It's a very cute little trick that is worth knowing, because in a very restricted set of cases, but not so restricted that it never comes up, it does make your life easier.

Unfortunately, as soon as you move beyond ratios of very simple expressions, taking derivatives usually makes symbolic representations of functions more complicated rather than simpler, so the rule may not really help you much. You're going to have to learn how to recover first-order approximations to functions anyway, and you might as well do that directly in the first step. Students who have l'Hôpital's rule as the only tool in their belt are going to be in trouble at this point.

For the case of the sine function, if you want to work on it from first principles, it's going to depend on how you define it in the first place. For definiteness, let's say we define sin θ as the y-coordinate of a point on the unit circle whose arc length coming up from the x-axis is θ.

Draw a triangle with one side being the line segment from the center of the circle to the intersection with the x-axis, one side is the segment out to the point on the circle at arc-length θ, and the third connects them. Draw another line from that point down to the x-axis. You should be able to see intuitively that, when θ is small, the lengths of those two small line segments are about the same, and about the same as θ. Now you'll need to actually prove it — iteratively bisecting the chord and showing that the error is small compared to θ should work. Granted it's more trouble than just plopping it into a cookbook formula, but it'll give you a lot more insight. --Trovatore (talk) 20:17, 6 August 2022 (UTC)

For this specific case, one can use (for small $\theta >0$) the inequality $\sin \theta <\theta <\tan \theta ,$ which can be rewritten equivalently as

$\cos \theta <{\frac {\sin \theta }{\theta }}<1.$

The result then follows from $\lim _{\theta \downarrow 0}\cos \theta =1.$  --Lambiam 20:48, 6 August 2022 (UTC)

Aside: That is a proper use of "begging the question"! So many people use "begs the question" improperly. Bubba73 ^{You talkin' to me?} 02:11, 7 August 2022 (UTC)

The answer to this question is "yes". --66.44.49.56 (talk) 14:25, 8 August 2022 (UTC)

You don't need to know that the derivative of sin(x) is cos(x), you just need to know that the derivative of sin(x) evaluated at x=0 is equal to 1. You should be able to prove that geometrically as User:Trovatore suggested. With that in hand, you can use the definition of the derivative to give the general result that the derivative of sin(x) is cos(x). --Amble (talk) 21:40, 11 August 2022 (UTC)