int main() {
    double x, x1, x2, dx, f0, f1, f2, f3;

    printf("xを入力して下さい。x = ");
    scanf("%lf", &x);

    dx = 0.01;
    x1 = x - dx;
    x2 = x + dx;

    f1 = ( f(x2) - f(x) )/dx;
    f2 = ( f(x) - f(x1) )/dx;
    f3 = ( f(x2) - f(x1) )/(2*dx);
    f0 = cos(x);

    printf("前進差分 f1 = %f\n", f1);
    printf("後退差分 f2 = %f\n", f2);
    printf("中心差分 f3 = %f\n", f3);
    printf("正確な値 f0 = %f\n", f0);
}