View Compilation Error

0_0_28090842_9894.cpp: In function 'int main()':
0_0_28090842_9894.cpp:2:21: error: 'acos' was not declared in this scope
 #define PI acos(-1.0)
                     ^
0_0_28090842_9894.cpp:10:13: note: in expansion of macro 'PI'
         t2=(PI*r)/v2;
             ^
0_0_28090842_9894.cpp:17:13: error: expected '}' at end of input
     return 0;
             ^