#include<stdio.h>
#include<math.h>
#define NJ 2
main()
{
   int i,j,k,p,n;
   float x[100],y[100],a[2][2],b[2],w[2];
   float s1,sx,sx2,sy,sxy;
   float pmax,s;
   printf("number of data n=");
   scanf("%d",&n);
   for(i=0;i<n;i++){
      printf("x[%d],y[%d]=",i+1,i+1);
      scanf("%f %f",&x[i],&y[i]);
   }
   /* */
   s1=(float)n;
   sx=0.0;
   sx2=0.0;
   sy=0.0;
   sxy=0.0;
   for(i=0;i<n;i++){
      sx+=x[i];
      sx2+=x[i]*x[i];
      sy+=y[i];
      sxy+=x[i]*y[i];
   }
   /* */
   a[0][0]=s1;
   a[0][1]=sx;
   a[1][0]=sx;
   a[1][1]=sx2;
   b[0]=sy;
   b[1]=sxy;

/* */
   printf("\n");
   for(i=0;i<NJ;i++){
      for(j=0;j<NJ;j++){
         printf("%10.2f",a[i][j]);
      }
      printf("%10.2f\n",b[i]);
   }
   printf("\n");

/* */

/* */

   for(k = 0; k < NJ-1; k++){
      p = k;
      pmax = fabs( a[k][k] );
      for(i = k+1; i < NJ; i++){
         if(fabs( a[i][k] ) > pmax){
            p = i;
            pmax = fabs( a[i][k] );
         }
      }

 /* */
      if(fabs( pmax ) < 1.0e-6){
         printf("too small pivot! \n");
         return(0);
      }
      if(p != k){
         for(i = k; i < NJ; i++){
            /* */
            s = a[k][i];
            a[k][i] = a[p][i];
            a[p][i] = s;
         }
         /* */
         s = b[k];
         b[k] = b[p];
         b[p] = s;
      }
/* */
      for(i = k +1; i < NJ; i++){
         w[i] = a[i][k] / a[k][k];
         a[i][k] = 0.0;
         /* */
         for(j = k + 1; j < NJ; j++){
            a[i][j] = a[i][j] - a[k][j] * w[i];
         }
         b[i] = b[i] - b[k] * w[i];
      }
   }
/* */
      for(i = NJ - 1; i >= 0; i--){
         for(j = i + 1; j < NJ; j++){
            b[i] = b[i] - a[i][j] * b[j];
            a[i][j] = 0.0;
         }
         b[i] = b[i] / a[i][i];
         a[i][i] = 1.0;
      }
/* */
      printf("\n");
      for(i = 0; i < NJ; i++){
         printf("x[%2d] = %12.8lf\n",i+1,b[i]);
      }
      printf("\n");
/* */
      printf("\n");
      printf("f(x)=%f + %f x\n",b[0],b[1]); 
      return(0);
}