View Compilation Error

0_0_15457501_31636.cpp: In function 'int main()':
0_0_15457501_31636.cpp:50:51: error: 'M_PI' was not declared in this scope
  printf("%.2lf\n", ((A+1)*a[0]*a[0] + B*a[0] + C)*M_PI);
                                                   ^
0_0_15457501_31636.cpp:72:24: error: 'M_PI' was not declared in this scope
    printf("%.2lf\n", k*M_PI);
                        ^