#include <stdio.h>
#include <math.h>

#define PI 3.141592653589793238462643383279

int main(void)
{
　　double r = PI / 6;
　　printf("sin(%.16f) = %.16f\n", r, sin(r));
　　r = asin(0.5);
　　printf("sin(%.16f) = %.16f\n", r, sin(r));
}

https://wandbox.org/permlink/ARy4fMg7s5j6Dubo
sin(0.5235987755982988) = 0.4999999999999999
sin(0.5235987755982989) = 0.5000000000000000

ラジアン派はこーゆーの気になんないの？