functions to calculate sin, cos, exp and pi in c

pi.c

// 3.1415926535897932384626433832795028841971693993751058209749445923

#include <stdio.h>
#include <stdint.h>

double pi_series_representation (int N)
{
  double result = 0.0;
  int8_t sign = 1;

  for (int n = 0; n <= N; ++n)
    {
      result += sign * (1.0 / (2 * n + 1));
      sign = -sign;
    }

  return 4 * result;
}

int main ()
{
  // still only gives 8 decimal places
  // there has to be a better way to calculate pi
  printf ("pi = %.20f\n", pi_series_representation (1000000000));

  return 0;
}

trig_functions_cos.c

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

#define DEG_TO_RAD(degree) ((degree) * M_PI / 180.0)

double cos_series_representation (double x, int N)
{
  double result = 1.0;
  double previous = 1.0; // first term of the sum

  for (int n = 1; n <= N; ++n)
    {
      double factor = -(x * x) / ((2 * n - 1) * (2 * n));
      double current = previous * factor;

      result += current;
      previous = current;
    }

  return result;
}

int main ()
{
  for (int degree = 0; degree <= 360; degree += 15)
    {
      printf ("sin(%3d°) = %.9f\n", degree,
              cos_series_representation (DEG_TO_RAD(degree), 100));
    }

  return 0;
}

trig_functions_exp.c

// e = 2.7182818284590452353602874713526624977572470936999595749669676277

#include <stdio.h>

double exp_series_representation (double x, int N)
{
  double result = 1.0;
  double previous = 1.0; // first term of the sum

  for (int n = 1; n <= N; ++n)
    {
      double factor = x / n;
      double current = previous * factor;

      result += current;
      previous = current;
    }

  return result;
}

int main ()
{
  printf ("exp(%.2f) = %.9f\n\n", 1.0,
          exp_series_representation (1.0, 100));
  printf ("exp(%.2f) = %.9f\n\n", 0.0,
          exp_series_representation (0.0, 100));
  printf ("exp(%.2f) = %.9f\n\n", -3.0,
          exp_series_representation (-3.0, 100));

  return 0;
}

trig_functions_sin.c

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

#define DEG_TO_RAD(degree) ((degree) * M_PI / 180.0)

double cos_series_representation (double x, int N)
{
  double result = x;
  double previous = x; // first term of the sum

  for (int n = 1; n <= N; ++n)
    {
      double factor = -(x * x) / ((2 * n) * (2 * n + 1));
      double current = previous * factor;

      result += current;
      previous = current;
    }

  return result;
}

int main ()
{
  for (int degree = 0; degree <= 360; degree += 15)
    {
      printf ("sin(%3d°) = %.9f\n", degree,
              cos_series_representation (DEG_TO_RAD(degree), 100));
    }

  return 0;
}

you are crazy