# How can I prove my calculator calculates a trigonometric function?

• Calculators

## Summary:

One book I have states that any scientific calculator works with this function: ##\arctan:\mathbb{R}\rightarrow{[-\pi,\pi]}##. I want to check it
Considering the measure of angles in radians, that are real numbers, the concept of of trigonometric function spreads to all real numbers. Any real number can be considered as an angle of the first circumference and a ##\mathbb{K}## number of circumferences.

We can consider the function ##\tan:\mathbb{R}\rightarrow{\mathbb{R}}##. This function is not bijective, but if we consider, instead of ##\mathbb{R}##, ##[-\pi,\pi]## as the set origin (which is what scientific calculators make), then it is bijective, and it's possible to define the inverse function ##\arctan:\mathbb{R}\rightarrow{[-\pi,\pi]}##

How can I check this function is which it works in my calculator?

Hope to have explained (and translated) well. I'm learning english, as you can clearly see. If it is not admissible, please delete it.

Greetings!

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PeroK
Homework Helper
Gold Member
2020 Award
Do you mean you want to calculate the inverse tangent of an number on your calculator and check your calculator gives the correct answer?

Wrichik Basu
I knew something was missing. No. I want to know which output does it calculate my device for the function ##\arctan##. Is it really ##[-\pi,\pi]##?

pbuk
Gold Member
I knew something was missing. No. I want to know which output does it calculate my device for the function ##\arctan##. Is it really ##[-\pi,\pi]##?
I doubt it. What answers does it give for ## \arctan 10\ 000 ## and ## \arctan -10\ 000 ## ?

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mcastillo356
PeroK
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I knew something was missing. No. I want to know which output does it calculate my device for the function ##\arctan##. Is it really ##[-\pi,\pi]##?
As they used to say in Scooby Doo: there's only one way to find out.

Although it should ##(-\pi/2, \pi/2)##.

Excel has no problem calculating ##\arctan##. Not sure why it should.

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mcastillo356
jedishrfu
Mentor
THis is an interesting and tough question to answer. We all trust our calculators to work correctly and accurately. Of course, there are some obscure failures that folks run into such as the Pentium bug or the mystery of ...

https://en.wikipedia.org/wiki/Pentium_FDIV_bug

https://en.wikipedia.org/wiki/Pentium_F00F_bug

or in Windows calculator when doing sqrt operations:

https://www.windowslatest.com/2018/...culator-bug-in-latest-windows-10-build-17639/

or this great video from Matt Parker on the ##11^6 / 13 = (156158413/3600) \pi## oddity on some calculators:

Lastly, for the historical record it was common practice for publishers of navigation tables to insert known mistakes to catch plagiarized tables from other publishers in England. However what was more likely was simple human error in calculating the values. It was one of the reasons why Babbage was commissioned to design and build his Difference engines.

https://www.npr.org/templates/story/story.php?storyId=121206408

Some other examples of fictitious entries to catch plagiarists:

https://en.wikipedia.org/wiki/Fictitious_entry

It may well happen that some calculator company might include an obscure bug to catch software theft. This idea was floated in the Matt Parker video too.

Jarvis323 and mcastillo356
Long live Charles Babbage.
##\arctan{10000}=1,570696327<\pi<2\pi##
##\arctan{-10000}=-1,570696327<-\pi>-2\pi##
Is this right? And if it's right, what does it mean?##[-\pi,\pi]## is the image of the function ##\arctan## for my calculator?

PeroK
PeroK
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2020 Award
Long live Charles Babbage.
##\arctan{10000}=1,570696327<\pi<2\pi##
##\arctan{-10000}=-1,570696327<-\pi>-2\pi##
Is this right? And if it's right, what does it mean?##[-\pi,\pi]## is the image of the function ##\arctan## for my calculator?
No, the image of ##\arctan## is ##(-\pi/2, \pi/2)##.

It can't be anything else.

mcastillo356
Yes, Perok, I'm rambling (I mean just wondering aimless). Yes, the image of ##\arctan## is ##(-\pi/2,\pi/2)##.
So I'd like to encourage you to wonder too: ##\arctan:\mathbb{R}\rightarrow{[-\pi,\pi]}##, is it the way scientific calculators work?. I mean, everyone. I have the feeling I'm asking you to do my homework. But, for example, ##\tan\pi/2=\mbox{Math ERROR}##, but is different, is ##+\infty##

PeroK
Homework Helper
Gold Member
2020 Award
So I'd like to encourage you to wonder too: ##\arctan:\mathbb{R}\rightarrow{[-\pi,\pi]}##, is it the way scientific calculators work?. I mean, everyone. I have the feeling I'm asking you to do my homework. But, for example, ##\tan\pi/2=\mbox{Math ERROR}##, but is different, is ##+\infty##
What comes out of a scientific calculator depends on how the calculor has been programmed/designed. But, I'd be surprised if any calculator gives you anything outside the range of ##(-\pi/2,\pi/2)##.

Also ##\tan\pi/2## is undefined, so Math ERROR is correct.

mcastillo356
jedishrfu
Mentor
When you write the interval as ##( -\pi/2, \pi/2)## it means the end points are not included in the interval also known as an open interval.

The calculator returning MATH ERROR is appropriate and correct.

I will ask for advice at the store, they will tell me how to manage.
Thanks, @PeroK, @jedishrfu, @pbuk!

jedishrfu
Mark44
Mentor
I will ask for advice at the store, they will tell me how to manage.
Better yet, look at the documentation for the calculator, which can be found on the manufacturer's website. If you go to the store, I'd bet that the people there would have no idea about the fine points about the calculator.

Tom.G, jedishrfu and mcastillo356
I bought a Casio fx-82MS this morning. The chinese clon is been left aside.
At the guideline, at the point 14. Function Calculations, it mentions ##\sin, \cos, \tan, \sin^{-1}, \cos^{-1}, \tan^{-1}##, and only adds: "Specify the angle unit before performing calculations".
Well, no problem, but the range of the result, how is it displayed? My question was if this range was ##[-\pi,\pi]##.
Later, it says:
"Pol, Rec: Pol converts rectangular coordinates to polar coordinates, while Rec converts polar coordinates to rectangular coordinates. And adds: "Specify the angle unit before performing calculations. Calculation result ##\theta## is displayed in the range of ##-180º<\theta\le{180º}##
My greyhound doesn't let me continue. I must carry her to the park. See you later

jedishrfu
Mentor
Greyhound Bus USA always had the motto: Go Greyhound ... and leave the driving to us.

I guess your dog has a similar motto.

mcastillo356
Her motto is: eat, run, sleep...an infinite loop. Now is sleeping, in a few minutes I will take her to the street to make the last walk before curfew, at 22:00 (in two hours): her name is Pepa

jedishrfu
jedishrfu
Mentor
We have a schipperke with similar habits: eat, run around the house like a greased pig, beg for treats at specific times of the day like clockwork, pester us to take him out as needed and also by 9pm and then pester us to go into his playpen at 10pm for a final treat before sleep.

mcastillo356
Fiendish, and lovely at the same time.

jedishrfu
pbuk
Gold Member
At the guideline, at the point 14. Function Calculations, it mentions ##\sin, \cos, \tan, \sin^{-1}, \cos^{-1}, \tan^{-1}##, and only adds: "Specify the angle unit before performing calculations".
The FX82 is capable of using angle units of degrees, radians or gradians. If I remember rightly you can switch between them by pressing SHIFT then MODE.

Well, no problem, but the range of the result, how is it displayed? My question was if this range was ##[-\pi,\pi]##.
The range for an inverse trig. function will never be ##[-\pi,\pi]##, see answers above.

"Pol, Rec: Pol converts rectangular coordinates to polar coordinates, while Rec converts polar coordinates to rectangular coordinates. And adds: "Specify the angle unit before performing calculations. Calculation result ##\theta## is displayed in the range of ##-180º<\theta\le{180º}##
And if you choose radians, then polar coordinate angles will be displayed in the range ##[-\pi,\pi]##, but that is not the same as inverse trig. angles.

My greyhound doesn't let me continue. I must carry her to the park. See you later
We say 'I need to take her to the park'. 'Carry' means that you hold her in your arms, and I don't think she will let you do that

The FX82 is capable of using angle units of degrees, radians or gradians. If I remember rightly you can switch between them by pressing SHIFT then MODE.
Yes.
In the USA you work with Texas Instruments calculators? I should have bought one of those, just to communicate with you better

The range for an inverse trig. function will never be [−π,π]##[-\pi,\pi]##, see answers above.
With this calculator, ##\arctan{10000}=1,570696327## in radians. ##\pi>1,570696327\approx{1,570796327}=\pi/2##
and ##\arctan{-10000}=-1,570696327\approx{-\pi/2}##
So, we have proved the range is ##[-\pi/2,\pi/2]##. It actually it tends to ##[-\pi/2,\pi/2]## when ##\mathbb{R}\rightarrow{\pm{\infty}}##

And if you choose radians, then polar coordinate angles will be displayed in the range [−π,π], but that is not the same as inverse trig. angles.
This last I haven't took a look. How do Pol and Rec work?

We say 'I need to take her to the park'. 'Carry' means that you hold her in your arms, and I don't think she will let you do that
Thanks for the remark about this word. I need to improve my english, and you help me.
Pepa wouldn't let me lift and carry her in her whole life. She's a greyhound, a barefooted princess.
Greetings! Sorry for the delay in answering: now it's 6:07 AM

Sorry, I'm being boring, I would like to know how Pol. and Rec. work. I know, for example, that Pol function calculates the polar coordinates of rectangular coordinates; but I introduce, for example, Pol(1,1), and I should obtain two integers for response: ##(r,\theta )##, but instead the modulus and the angle, I only obtain ##1,414213562=\sqrt{2}=r##; How must I type it to obtain both ##r## and ##\theta##?

For all I know, the logic gates might all be designed around trigonometry so the jump in logic is in calculating things like 1+1. Q:?)

I found it!
Example: Convert rectangular ##(1,\sqrt{3})## to polar ##(r,\theta)## (Rad)
##r=2## ##\rightarrow´{}## ##\boxed{Pol(}## ##1## ##,## ##\sqrt{}## ##3## ##)## ##=##
##\theta=1,047197551## ##\rightarrow´{}## ##\boxed{RCL}## ##\mbox{F}##
Press ##\boxed{RCL}## ##\mbox{E}## to display the measure of ##r##, or ##\boxed{RCL}## ##\mbox{F}## for the angle ##\theta##
So...I don't recommend this scientific calculator.
Greetings, UppercaseQ, pbuk, jedishrfu, PeroK!

pbuk
Gold Member
In the USA you work with Texas Instruments calculators? I should have bought one of those, just to communicate with you better
They might do but I'm in the UK where Casio calculators are far more popular as they are in most of Europe I think (although sometimes rebadged under local brands with keys marked in local languages).

With this calculator, ##\arctan{10000}=1,570696327## in radians. ##\pi>1,570696327\approx{1,570796327}=\pi/2##
and ##\arctan{-10000}=-1,570696327\approx{-\pi/2}##
So, we have proved the range is ##[-\pi/2,\pi/2]##.
Yes indeed; I think it is the same for the other inverse trig. functions. (Edit: strictly speaking the range for arctan is ## (-\frac{\pi}2, \frac{\pi}2) ## because ## \tan \frac{\pi}2 ## is undefined).

Sorry, I'm being boring, I would like to know how Pol. and Rec. work. I know, for example, that Pol function calculates the polar coordinates of rectangular coordinates; but I introduce, for example, Pol(1,1), and I should obtain two integers for response: ##(r,\theta )##, but instead the modulus and the angle, I only obtain ##1,414213562=\sqrt{2}=r##; How must I type it to obtain both ##r## and ##\theta##?
Not two integers, the modulus is the real number ## \sqrt 2 ##. The display on the FX82 can only display one number at a time so...

I found it!
Example: Convert rectangular ##(1,\sqrt{3})## to polar ##(r,\theta)## (Rad)
##r=2## ##\rightarrow´{}## ##\boxed{Pol(}## ##1## ##,## ##\sqrt{}## ##3## ##)## ##=##
##\theta=1,047197551## ##\rightarrow´{}## ##\boxed{RCL}## ##\mbox{F}##
Press ##\boxed{RCL}## ##\mbox{E}## to display the measure of ##r##, or ##\boxed{RCL}## ##\mbox{F}## for the angle ##\theta##
... that is how you display the angle.

So...I don't recommend this scientific calculator.
It's a good calculator and was for some time the one most widely used in UK schools (although this was partly because more advanced calculators were not permitted). Its main disadvantage as you have found is that it can only display one number at a time; calculators with multi-line displays like the FX-85 are easier to work with.

mcastillo356