View Compilation Error

0_0_35744883_20964.cpp: In function 'int main()':
0_0_35744883_20964.cpp:11:42: error: 'M_PI' was not declared in this scope
             sum+=len*len*sin(ang/180.0 * M_PI)*0.5;
                                          ^
0_0_35744883_20964.cpp:11:46: error: 'sin' was not declared in this scope
             sum+=len*len*sin(ang/180.0 * M_PI)*0.5;
                                              ^