Numerical Methods: Parabolic curve fitting using C

Source Code:

#include<stdio.h>
#include<math.h>
int main(){
    float xy[20][20], matrix[3][4], ratio, a;
    float sum_x = 0, sum_y = 0, sum_x2 = 0, sum_x3 = 0, sum_x4 = 0, sum_xy = 0, sum_x2y = 0;
    int i, j , k, n;
    printf("Enter no of data: ");
    scanf("%d", &n);
    printf("Enter the data: \n");
    for(i = 0; i < 2; i++){
        for(j = 0; j < n; j++){
            scanf("%f", &xy[i][j]);
        }
    }
    for(i = 0; i < n; i++){
        sum_x += xy[0][i];
        sum_y += xy[1][i];
        sum_x2 += pow(xy[0][i], 2);
        sum_x3 += pow(xy[0][i], 3);
        sum_x4 += pow(xy[0][i], 4);
        sum_xy += xy[0][i]*xy[1][i];
        sum_x2y += pow(xy[0][i], 2) * xy[1][i];
    }
    matrix[0][0] = n;
    matrix[0][1] = sum_x;
    matrix[0][2] = sum_x2;
    matrix[0][3] = sum_y;
    matrix[1][0] = sum_x;
    matrix[1][1] = sum_x2;
    matrix[1][2] = sum_x3;
    matrix[1][3] = sum_xy;
    matrix[2][0] = sum_x2;
    matrix[2][1] = sum_x3;
    matrix[2][2] = sum_x4;
    matrix[2][3] = sum_x2y;
    for(i = 0; i < 3; i++){
        for(j = 0; j < 3; j++){
            if(i!=j){
                ratio = matrix[j][i]/matrix[i][i];
                for(k = 0; k < 4; k++){
                    matrix[j][k] -= ratio * matrix[i][k];
                }
            }
        }
    }
    for(i = 0; i < 3; i++){
        a = matrix[i][i];
        for(j = 0; j < 4; j++){
            matrix[i][j] /= a;
        }
    }
    for(i = 0; i < 3; i++){
        printf("\n%c => %.2f", 97+i, matrix[i][3]);
    }
}

Numerical Methods: Integration of given function using Trapezoidal rule in C

Source Code:

///integration of given function using Trapezoidal rule
#include<stdio.h>
float y(float x){
    return 1/(1+x*x);
}
int main(){
    float x0,xn,h,s;
    int i,n;
    printf("Enter x0, xn, no. of subintervals: ");
    scanf("%f%f%d",&x0,&xn,&n);
    h = (xn-x0)/n;
    s = y(x0) + y(xn);
    for(i = 1; i < n; i++){
        s += 2*y(x0+i*h);
    }
    printf("Value of integral is %6.4f\n",(h/2)*s);
    return 0;
}