The program is a bit simpler looking with a for loop.
float trapezoid (float a, float b, int N) {
float sum, h;
int i;
sum = integrand(a)/2;
h = (b-a) / N;
for (i = 1; i < N; i = i+1) {
sum = sum + integrand(a + i*h);
}
sum = sum + integrand(b)/2;
return(sum*h);
}