View Compilation Error

0_0_13214636_26102.cpp: In function 'int main()':
0_0_13214636_26102.cpp:2:21: error: 'acos' was not declared in this scope
 #define PI acos(-1.0)
                     ^
0_0_13214636_26102.cpp:30:30: note: in expansion of macro 'PI'
      printf("%.4f\n",mid*mid*PI);
                              ^