/in/foo.cc: In function 'int main()':
/in/foo.cc:10:27: warning: unused variable 'x4' [-Wunused-variable]
  double x1,y1,x2,y2,x3,y3,x4,y4,a,b,c;
                           ^~
/in/foo.cc:10:30: warning: unused variable 'y4' [-Wunused-variable]
  double x1,y1,x2,y2,x3,y3,x4,y4,a,b,c;
                              ^~