Re: [PATCH guile-lib] src: (math trigonometry): Add it.

On 16/02/2025 12:19, Maxime Devos wrote:

>> I'm going to look into what's the case for 'tan'. For 'tan', 
>> tan(pi/4)=tan(45°) is rational as well - I'm going to look into 
>> whether the 'S' of 'tan' is only the integer multiples pi/4, or 
>> whether there are more.
>>
> It's only those:
>
> For which θ is θ/π and tan(θ) rational?
> Assume t=tan(θ)∈ℚ. Note that sin (2θ)=(2t)/(1+t^2)∈ℚ. Since sin(2θ) is 
> rational and 2θ/π is rational, then 2θ is an integer multiple of π/2. 
> So, θ is an integer multiple of π/4.
>
This is untrue, there are a few other angles (that are rational 
multiples of pi) for which sin is rational. The same approach should 
work, though.