View Compilation Error

0_0_31378149_32335.cpp: In function 'int main()':
0_0_31378149_32335.cpp:16:60: error: 'f' was not declared in this scope
   y = a - pow(a, 3) / 6 + pow(a, 5) / 120 - pow(a, 7) / f(7) + pow(a, 9) / f(9);
                                                            ^