Hi I want to implement the trpezoidal rule which is explained as follows. I have written the code for it but it seems to give me the wrong output. Please help.

The code is as follows:
#include <stdio.h>

double trapezoidal(int a,int b,int c,int d,double lowx,double highx,int intervals);
main()
{
 int w,x,y,z;
 double x1,x2,answer1,answer2,answer3;
 printf("Enter the first integer => ");
 scanf("%d",&w);
 printf("Enter the second integer => ");
 scanf("%d",&x);
 printf("Enter the third integer => ");
 scanf("%d",&y);
 
 printf("Enter the fourth integer => ");
 scanf("%d",&z);
 
 printf("\n");printf("\n");
 puts("Enter values of start and end of interval ");
 printf("\n");
 printf("Enter the starting value => ");
 scanf("%lf",&x1);
 
 printf("Enter the  ending value=> ");
 scanf("%lf",&x2);
 answer1=trapezoidal(w,x,y,z,x1,x2,100);
 answer2=trapezoidal(w,x,y,z,x1,x2,1000);
 answer3=trapezoidal(w,x,y,z,x1,x2,10000);
 puts("Intervals   Area"); printf("\n");
 printf("  100 %f \n",answer1);
 printf(" 1000 %f \n",answer2);
 printf("10000   %f \n",answer3); printf("\n");
}

double trapezoidal(int a,int b,int c,int d,double lowx,double highx,int intervals)
{
 double answer,answer1,answer2,answer3,X,f,Y,Z;
 int i;
 X=(highx-lowx)/(2*intervals);
 Y=(highx-lowx)/intervals;
 Z=lowx+Y;
 answer1=a*lowx*lowx*lowx - b*lowx*lowx + c*lowx +d;
 for(i=Z; i<=highx; i=i+Y)
 {     
   f=a*i*i*i-b*i*i+c*i+d;
   answer2=2*f;
 }
 answer3=a*highx*highx*highx - b*highx*highx + c*highx + d;
 answer=X*(answer1+answer2+answer3);
 return(answer); printf("\n");
}



The Trapezoidal Rule.

Integrating f(x)dx over the interval a to b=

      ((b-a)/(2n))[f(x0) + 2f(x1)+ 2f(x2) + 2(fx3) +..... +2f(x(n-1))+f(xn)]

    0,1,2......n are the intervals. The number of intervals is the number of trapezoids.

 The width of every trapezoid is (b-a)/n.

  a=x0<x1<x2<x3......<xn=b means that the value of x increases from a to b in the interval and as I understand it increases (b-a)/n times.