void trapzd(double a, double b, double& s, int n)
{
	double del,x,sum;
	int j,it, tnm;
    if (n ==1)
	{
        s = 0.5 * (b - a) * (func(a) + func(b));
        it = 1;
	}
    else
	{
        it = (int)pow(2 , n - 2);
        tnm = it;
        del = (b - a) / tnm;
        x = a + 0.5 * del;
        sum = 0.0;
        for (j = 1; j<=it; j++)
		{
            sum = sum + func(x);
            x = x + del;
        }
        s = 0.5 * (s + (b - a) * sum / tnm);
    }
}