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

static long fat = 1; // fat(2*0)

int main()
{
  int n, i, j;
  double x, z = 0.0;

  scanf("%lf%d", &x, &n);

  for (i = 0; i < n; i++ )
  {
    printf("* i=%d fat(%d)=%dn", i, 2*i, fat);
    z = z + ( ( pow(-1.0 , i)*pow(x, (2.0*i)) ) / fat );
    fat = fat * (2*i+1) * (2*i+2); // proximo fat
  }

  printf("nncos(x) = %lfn", z );

  return 0;
}

[~/Projects/testes/so]
$./377309
3 5
* i=0 fat(0)=1
* i=1 fat(2)=2
* i=2 fat(4)=24
* i=3 fat(6)=720
* i=4 fat(8)=40320

cos(x) = -0.974777

[~/Projects/testes/so]
$