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/////////////////////////////////////////////////////////////////////
//                Purpose: (x_i,y_i)線性擬合函數                   //
/////////////////////////////////////////////////////////////////////

#include <stdio.h>
#define MAX_N       25

typedef struct tagPOINT
{
  double x;
  double y;
} POINT;

int main()
{
  int m;
  int i;
  POINT points[MAX_N];
  static double u11, u12, u21, u22, c1, c2;
  double a, b, tmp;
  printf("\n Input m value: ");
  scanf("%d", &m);
  if (m > MAX_N)
  {
    printf("The input m is larger than MAX_N, please redefine the MAX_N.\n");
    return 1;
  }
  if (m <= 0)
  {
    printf("Please input a number between 1 and &d. \n", MAX_N);
    return 1;
  }
  printf("Now input the (x_i, y_i), i=0, ..., %d: \n", m - 1);
  for (i = 0; i < m; i++)
  {
    scanf("%lf", &tmp);
    points[i].x = tmp;
    scanf("%lf", &tmp);
    points[i].y = tmp;
  }
  for (i = 0; i < m; i++)
  {
    u21 += points[i].x;
    u22 += points[i].x *points[i].x;
    c1 += points[i].y;
    c2 += points[i].x *points[i].y;
  }
  u12 = u21;
  u11 = m;

  a = (c1 *u22 - c2 * u12) / (u11 *u22 - u12 * u21);
  b = (c1 *u21 - c2 * u11) / (u21 *u12 - u22 * u11);
  printf("Solve: p(x)=%f+%fx\n", a, b);
  system("pause");
  return 0;
}