График функции на языке Си

# include <stdio.h>
# include <conio.h>
# include <math.h>
# include <stdlib.h>
# include <iostream.h>
# include <graphics.h>
# include <dos.h>
float S, x0, x1, x2, a, b, c, h;
float y0(float x0);
float y1(float x1);
float y2(float x2);
float y0(float x0) { (float)(a*(pow(x0,3))+(b*(x0))+c); return (a*(pow(x0,3))+(b*(x0))+c); }
float y1(float x1) { (float)((a*(pow(x1,3))+(b*(x1))+c)*4); return ((a*(pow(x1,3))+(b*(x1))+c)*4); }
float y2(float x2) { (float)(a*(pow(x2,3))+(b*(x2))+c); return (a*(pow(x2,3))+(b*(x2))+c); }
 
  void main()
{
int i, n, L, R;
textbackground (0);
textcolor (15);
clrscr();
printf ("BBeguTe LeBylu GpaHucy (oT -1000 go 1000): "); // BBog LeBoi GpaHucbl
scanf ("%d", &L);
printf ("BBeguTe npaByu GpaHucy (oT -1000 go 1000): ");
scanf ("%d", &R);               // BBog npaBoi GpaHucbl
if ((L<-1000) || (L>1000) || (R<-1000) || (R>1000))
{
      printf ("HeBo3Mo}l{Ho BblnoLHeHue, BBeguTe GpaHucy oT -1000 go 1000\n");
      printf("Press any key to halt:");
      getch();
      exit(1); 
   }
printf ("BBog a: ");
scanf ("%f", &a);
printf ("BBog b: ");
scanf ("%f", &b);
printf ("BBog c: ");
scanf ("%f", &c);
printf ("BBeguTe 4ucLo Pa36ueHui(oT 0 go 500): ");
scanf ("%d", &n);
if ((n<0) || (n>500))
{
     printf ("BBeguTe 4ucLo Pa3bueHui oT 0 go 500\n");
     printf("Press any key to halt:");
     getch();
     exit(1); 
}
h=(float)(R-L)/n;
x0=L;
x1=L+h;
x2=L+2*h;
{
     for (i=1; i<=(n/2); i+=1)
{
  S+=(float)(h/3)*(y0(x0)+y1(x1)+y2(x2));
  x0+=2*h;
  x1+=2*h;
  x2+=2*h;
}
}
printf ("S(ax^3+bx+c)=%f\n", S);
getch();
}