三分~~~~

注意第7行即可

void solve(double l,double r) {
	//l左边界，r右边界 
	double eps = 1e-12;//精度 
	while (r - l > eps) {
		double mid = (l + r) / 2.0;
		double mmid = (mid + r) / 2.0;
		if (f(mid) < f(mmid))//计算最小值用<,若为最大值则>
			r = mmid;
		else
			l = mid;
	}
	cout<<r;
}