> 各種言語のライブラリを作ってるのはたいていラジアン派だがお前のようなアホは一人もいないぞ

このひとBoostとか.NET Frameworkとか知らないんだろうな。

#include <iostream>
#include <boost/math/special_functions/sin_pi.hpp>
#include <boost/math/special_functions/cos_pi.hpp>

int main()
{
　　for (int i = 0; i <= 8; i++) {
　　　　double s = boost::math::sin_pi(i / 4.0);
　　　　double c = boost::math::cos_pi(i / 4.0);
　　　　printf("sin(%d/4π) = %19.16f cos(%d/4π) = %19.16f\n", i, s, i, c);
　　}
}

sin(0/4π) = 0.0000000000000000 cos(0/4π) = 1.0000000000000000
sin(1/4π) = 0.7071067811865476 cos(1/4π) = 0.7071067811865476
sin(2/4π) = 1.0000000000000000 cos(2/4π) = 0.0000000000000000
sin(3/4π) = 0.7071067811865476 cos(3/4π) = -0.7071067811865476
sin(4/4π) = -0.0000000000000000 cos(4/4π) = -1.0000000000000000
sin(5/4π) = -0.7071067811865476 cos(5/4π) = -0.7071067811865476
sin(6/4π) = -1.0000000000000000 cos(6/4π) = 0.0000000000000000
sin(7/4π) = -0.7071067811865476 cos(7/4π) = 0.7071067811865476
sin(8/4π) = 0.0000000000000000 cos(8/4π) = 1.0000000000000000

https://wandbox.org/permlink/IEFafqP4uhbG2QWv