View Compilation Error

0_0_16187438_23398.cpp: In function 'double DEG(double)':
0_0_16187438_23398.cpp:149:42: error: 'M_PI' was not declared in this scope
 double DEG(double x) { return (180.0*x)/(M_PI*1.0);}
                                          ^
0_0_16187438_23398.cpp: In function 'double RAD(double)':
0_0_16187438_23398.cpp:150:35: error: 'M_PI' was not declared in this scope
 double RAD(double x) { return (x*(M_PI*1.0))/(180.0);}
                                   ^