cos.c

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

double fact_rev(int n) {
  double result = 1.0;
  for (int i = 1; i <= n; ++i) {
    result *= 1.0 / i;
  }
  return result;
}

double my_cos(double x) {
  double ans = 0;
  for (int i = 0; i <= 20; ++i) {
    ans += pow(-1, (double)i) * fact_rev(2 * i) * pow(x, 2 * (double)i);
  }
  return ans;
}

int main() {
  printf("%f\n", my_cos(M_PI / 2));
}