View Compilation Error

0_0_33754134_30113.cpp:2:26: error: 'acos' was not declared in this scope
 const double PI = acos(-1);
                          ^
0_0_33754134_30113.cpp: In function 'int main()':
0_0_33754134_30113.cpp:10:27: error: 'sqrt' was not declared in this scope
         a=sqrt(x1*x1+y1*y1);
                           ^
0_0_33754134_30113.cpp:14:19: error: 'acos' was not declared in this scope
         w = acos(w) * 180 / PI;
                   ^