use of undeclared identifier 'sqrt'? [closed]

In my C++11 code I have:

#include <functional>
#include <string>
#include <iostream>
#include <sstream>
#include <ostream>
#include <map>
#include <iterator>
#include <string>
#include <fstream>

#include "Matrix.h"

#define DEPENDENCY_VERBOSE

using namespace mtm;
using namespace std;
using std::cout;
using std::endl;
using std::string;
using std::map;
using std::pair;
using std::ostream;

But when I write:

double x=sqrt(5);

I get:

use of undeclared identifier 'sqrt'

How to fix this?