View Compilation Error

0_0_39383409_13766.cpp: In function 'double sanfen(double, double)':
0_0_39383409_13766.cpp:14:14: error: 'lmid' was not declared in this scope
   14 |        if( F(lmid)<F(rmid) ) //凹点落在左半区--更新右边界
      |              ^~~~
0_0_39383409_13766.cpp:14:22: error: 'rmid' was not declared in this scope
   14 |        if( F(lmid)<F(rmid) ) //凹点落在左半区--更新右边界
      |                      ^~~~
0_0_39383409_13766.cpp: In function 'int main()':
0_0_39383409_13766.cpp:24:5: error: 'cin' was not declared in this scope; did you mean 'std::cin'?
   24 |     cin>>t
      |     ^~~
      |     std::cin
In file included from 0_0_39383409_13766.cpp:1:
gcc-13.1.0/lib/gcc/x86_64-w64-mingw32/13.1.0/include/c++/iostream:62:18: note: 'std::cin' declared here
   62 |   extern istream cin;           ///< Linked to standard input
      |                  ^~~