double chebev(double a, double b, double c[], int m, double x)
{
    if ((x - a) * (x - b) > 0.0)
	{
		cout<<"x not in range."<<endl;
	}
    double d = 0.0;
    double sv,y,y2,dd = 0.0;
    y = (2.0 * x - a - b) / (b - a);
    y2 = 2.0 * y;
    for (int j = m; j>=2; j--)
	{
        sv = d;
        d = y2 * d - dd + c[j];
        dd = sv;
    }
    return y * d - dd + 0.5 * c[1];
}