/in/foo.cc: In function 'int main()':
/in/foo.cc:12:49: error: 'sqrt' was not declared in this scope
  double len=sqrt((a1-a2)*(a1-a2)+(b1-b2)*(b1-b2));
                                                 ^
/in/foo.cc:14:26: error: 'sin' was not declared in this scope
  double ai=len*sin(18*DEC);
                          ^
/in/foo.cc:15:26: error: 'cos' was not declared in this scope
  double bi=len*cos(18*DEC);
                          ^