View Compilation Error

0_0_25447879_18118.cpp: In function 'bool pri(int)':
0_0_25447879_18118.cpp:21:19: error: 'sqrt' was not declared in this scope
  for(i=2;i<=sqrt(k);i++)
                   ^
0_0_25447879_18118.cpp:24:14: error: 'sqrt' was not declared in this scope
   if(i>sqrt(k))
              ^