double erfen(double left,double right)
{
	while(right-left>=precisity)
	{
		...
	}
	return left;
}

会错,改成

double erfen(double left,double right)
{
	while(right-left>=precisity)
	{
		...
	}
	return right;
}

就AC了，调精度也没用，为什么？