Interesting approximations using trigonometry

Today on Twitter John D. Cook, writing as @AlgebraFact, posted the following tweet:

In radians, sin(11) is very nearly -1.

(It happens to be -0.9999902…)

I thought that was awesome! So, I (@divbyzero) replied that cos(333) is approximately 1. (It is 0.999961…)

Then @michiexile chimed in, pointing out that cos(355) is closer to -1 than cos(333) is to 1. (It is -0.9999999995…)

Finally, I countered with cos(103993), which is 0.9999999998…

So what’s going on here?

All of this comes from the continued fraction for $\pi$ ,

${\pi =3+\cfrac {1}{7+\cfrac {1}{15+\cfrac {1}{1+\cfrac {1}{292+\cfrac {1}{1+\cdots }}}}}}$ .

It is well known that the convergents of a continued fraction (i.e., the fraction you obtain by truncating a continued fraction at some point) are the best rational approximations for the number. The first ten convergents of ${\pi }$ are

${3}$ , $\frac{22}{7}$ , $\frac{333}{106}$ , $\frac{355}{113}$ , $\frac{103993}{33102}$ , $\frac{104348}{33215}$ , $\frac{208341}{66317}$ , $\frac{312689}{99532}$ , $\frac{833719}{265381}$ , $\frac{1146408}{364913}$ .

Notice that $\pi \approx \frac{22}{7}$ , so $11\approx \frac{7\pi }{2}$ . Thus $\sin (11)\approx \sin \big (\frac{7\pi }{2}\big )=-1$ . This was John D. Cook’s observation.

Similarly, $\pi \approx \frac{333}{106}$ , so $333\approx 106\pi$ , and ${\cos (333)\approx \cos (106\pi )=1}$ . The other two approximations come from the next two convergents.

After the conversation on Twitter, I started playing a little more.

First, I noticed that

${\tan (111)\approx \sqrt {3}}$ .

( ${\tan (111)-\sqrt {3}\approx -0.011\ldots }$ )

This comes from the third convergent. Since $\pi \approx \frac{333}{106}$ implies that $111\approx 2\pi \cdot 17+\frac{4\pi }{3}$ , $\tan (111)\approx \tan \big (2\pi \cdot 17+\frac{4\pi }{3}\big )=\tan \big (\frac{4\pi }{3}\big )=\sqrt {3}$ .

Actually, we can do better than that:

$-\tan(69447)\approx \sqrt{3}$ .

( $-\tan(69447)-\sqrt{3}\approx-0.000011\ldots$ )

The approximation, $\displaystyle \pi\approx\frac{208341}{66317}$ implies that $\displaystyle 69447\approx 2\pi\cdot66317+\frac{5\pi}{3}$ , so $\displaystyle \tan(69447)\approx \tan\big(2\pi\cdot66317+\frac{5\pi}{3}\big)=\tan\big(\frac{5\pi}{3}\big)=-\sqrt{3}$ .

Next, I noticed that

${\sec (26087)\approx \sqrt {2}}$ .

( ${\sec (26087)-\sqrt {2}\approx -0.0000039\ldots }$ )

This comes from $\pi \approx \frac{104348}{33215}$ , which implies that $26087\approx 2\pi \cdot 4151+\frac{7\pi }{4}$ . Thus $\sec (26087)\approx \sec \big (2\pi \cdot 4151+\frac{7\pi }{4}\big )=\sec \big (\frac{7\pi }{4}\big )=\sqrt {2}$

Finally, I noticed that

${-\sec (26087)-\sec (95534)\approx \sqrt {6}}$ .

( ${-\sec (26087)-\sec (95534)-\sqrt {6}\approx 0.0000046\ldots }$ )

In this case, $\pi \approx \frac{1146408}{364913}$ gives $95534\approx 2\pi \cdot 15204+\frac{17\pi }{12}$ , and $\sec (95534)\approx \sec \big (2\pi \cdot 15204+\frac{17\pi }{12}\big )=\sec \big (\frac{17\pi }{12}\big )=-\sqrt {6}-\sqrt {2}$ . We obtain the result by substituting the previous approximation for ${\sqrt {2}}$ .

Of course, we also have

$\sqrt{6}=\sqrt{2}\sqrt{3}\approx -\sec(26087)\tan(69447)$ .

( $-\sec(26087)\tan(69447)-\sqrt{6}=0.0000086\ldots$ )

14 Comments

John says:

February 17, 2010 at 12:17 am

That’s great! You really ran with this.

One footnote: Good approximations to pi lead to great approximations to 1 when you take sines. Taylor series says sin(pi/2 + h) is approximately 1 – 0.5 h^2 for small h. So the error in the approximation for pi gets squared.

Dave Richeson says:

February 17, 2010 at 11:06 am

John,
Thanks for pointing that out. I did notice that some approximations were better than others, but hadn’t thought about why.

Dave

Nemo says:

February 17, 2010 at 3:27 pm

Reminds me of my favorite calculator trick.

Set your calculator to degrees mode (NOT radians).

Type in a bunch of 5’s: 555555, or whatever.

Press “1/x”.

Press “sin”.

Examine the mantissa of the result. Magic!

Dave Richeson says:

February 17, 2010 at 3:38 pm

Excellent, I’d never seen that before! Good use of $\sin x\approx x$ and the radians-to-degrees conversion. I’ll have to show that to my students.

Pingback: The math behind a neat calculator trick « Division by Zero

Walt says:

February 24, 2010 at 9:06 am

Found a small typo in your typesetting: the CF of pi is 3; 7, 15, 1, 292 – you forgot the 1 after the 15 in your nested fraction.

Dave Richeson says:

February 24, 2010 at 9:51 am

Wow, good eyes. Thank you for catching that. I updated the post.

Pingback: Approximations with continued fractions « Alasdair's musings

Prasad Kulkarni says:

March 7, 2011 at 1:51 am

Awesome.. You guys really doing some good approximation and bringing some fun to it. How I wish Twitter had been there in my school days.

Ragesh G R says:

February 3, 2012 at 8:29 am

Hi, seems like you have done gross approximations.

Tan 4pi/3 is no way near sec 4pi/3 . Same goes for 5pi/3. Unless i am missing something :)

Dave Richeson says:

February 5, 2012 at 9:13 pm

Thanks for catching the typos. They’re fixed now.

batbrat says:

February 3, 2012 at 8:40 am

I read through the post several times and discussed it with my friends. I am unable to understand why have you approximated tan(111) using sec(111) and tan(69447) using sec(69447).

This is especially puzzling when I consider that computing sec(4*pi/3) in radians yields -2 and tan(4/3 * pi) is 1.7320! They aren’t even close! I’m utterly puzzled.

I’d really appreciate some insight into your thought process.

On an aside, I’d much appreciate it if you deleted my previous comment. I can’t really edit my comment, so I re-posted.

Dave Richeson says:

February 5, 2012 at 9:14 pm

Thanks for catching those typos.
1. batbrat says:
  
  February 6, 2012 at 7:50 am
  
  No problem. :)

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