sine C program with Power Series

I don't what's wrong with this but the result is always 0.0

#include "poly.h"
#include <math.h>
/*
* polyEval: Evaluates p(x) = a[0] + a[1] * x + ... + a[n] * x^n.
* Pre: n >= 0 and array a[] has at least n+1 elements.
*/
double polyEval(double *a, int n, double x)
{
int i; /* counter for the array */
double sum=0.0;

for (i=0; i<=n; i++){
sum+= a[i]*pow(x,i);}
return sum;
}

/*
* sinePoly: Evaluates sin(x) using the first numTerms (nonzero) terms of
* its infinite series.
* Pre: 1 <= numTerms <= MAXTERMS.
*/
double sinePoly(double x, int numTerms)
{
double a[MAXTERMS];

while (numTerms >= 0){
a[numTerms]=*blah calculations*;
numTerms--;
}
return polyEval(a, numTerms, x);
}

Maybe it's like that 'cause...
in Your last starement 'return polyEval(a, numTerms, x)' in function 'double sinePoly(double x, int numTerms)' you call 'polyEval(...)' with parameter 'numTerms' which gets counted down to 0 in 'while (numTerms >= 0){...}' loop. So the loop 'for (i=0; i<=n; i++){...}' in 'double polyEval(double *a, int n, double x)' gets only executed once.

<!Just a guess!>