foo.cc: In function 'int main()':
foo.cc:8:16: error: 'ln' was not declared in this scope
  printf("%.5f",ln(x/y)+1);
                ^~
foo.cc:8:16: note: suggested alternative: 'yn'
  printf("%.5f",ln(x/y)+1);
                ^~
                yn
foo.cc:7:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%lf%lf",&x,&y);
  ~~~~~^~~~~~~~~~~~~~~~

自豪的采用 HydroJudge 进行评测(github.com/hydro-dev/HydroJudge)