0_0_36030209_13559.cpp: In function 'int main()': 0_0_36030209_13559.cpp:9:41: error: 'M_PI' was not declared in this scope cout<<fixed<<setprecision(6)<<sin(2*M_PI/n)/2*(n-1)+sqrt((2-2*cos(2*M_PI/n)))/2<<endl; ^