View Compilation Error

1_866_2126_31783.cpp: In function 'double msin(double)':
1_866_2126_31783.cpp:5:16: error: 'M_PI' was not declared in this scope
     return sin(M_PI * d / 180.0);
                ^
1_866_2126_31783.cpp: In function 'double mcos(double)':
1_866_2126_31783.cpp:8:16: error: 'M_PI' was not declared in this scope
     return cos(M_PI * d / 180.0);
                ^