Tính gần đúng tích phân(Simpson)

//Tinh tich phan bang cong thuc Simpson
#include <stdio.h>
#include <conio.h>

float f(float x)
{
return (15-2*x)/(3*x+7.2);
}

void main()
{
float a;
float b;
int n;
float h;
float S=0;
int i;

clrscr();
printf("a=");
scanf("%f",&a);
while (3*a+7.2==0)
{
printf("\Khong hop le!Nhap lai");
printf("\na=");
scanf("%f",&a);
}

printf("b=");
scanf("%f",&b);
while (3*b+7.2==0)
{
printf("\Khong hop le!Nhap lai");
printf("\nb=");
scanf("%f",&b);
}

printf("\nn=");
scanf("%d",&n);
while (n%2!=0)
{
printf("\nn phai la so chan!Nhap lai!");
printf("\nn=");
scanf("%d",&n);
}
h=(b-a)/n;

for (i=0;i<=n-2;i=i+2)
S+=(f(a+i*h)+4*f(a+(i+1)*h)+f(a+(i+2)*h))*h/3;

printf("\nS=%f",S);
getch();
}