View Compilation Error

0_0_35795071_4026.cpp: In function 'double jl(double, double, double, double)':
0_0_35795071_4026.cpp:10:56: error: 'sqrt' was not declared in this scope
  a = sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2));
                                                        ^
0_0_35795071_4026.cpp: In function 'void yx(double, double, double, double)':
0_0_35795071_4026.cpp:21:22: error: 'sqrt' was not declared in this scope
  s = sqrt(1.0 - s * s);
                      ^