View Compilation Error

0_0_34182883_17819.cpp: In function 'int main()':
0_0_34182883_17819.cpp:12:41: error: 'acos' was not declared in this scope
    area += r * r * sin(k / 180 * acos(-1)) / 2;
                                         ^
0_0_34182883_17819.cpp:12:42: error: 'sin' was not declared in this scope
    area += r * r * sin(k / 180 * acos(-1)) / 2;
                                          ^