View Compilation Error

0_0_34070161_32332.cpp: In function 'int main()':
0_0_34070161_32332.cpp:3:19: error: 'acos' was not declared in this scope
 #define PI acos(-1);
                   ^
0_0_34070161_32332.cpp:15:30: note: in expansion of macro 'PI'
             V[i]=radii*radii*PI;
                              ^