permutation and combination operation

src/main/c/Calculator/calculator.c

@@ -15,6 +15,8 @@ double square(double num);
 double sine(double angle);
 double cosine(double angle);
 double tangent(double angle);
+double permutation(int n, int r);
+double combination(int n, int r);
 
 int main() {
     double choice, num1, num2, result, angle;