Code

 Header...
//
// Demonstrate the trigonometric identity
// cos(a+b) equals cos(a)*cos(b) - sin(a)*sin(b)
//

 int main(){
   ... 

»  double a, b, cos_a_plus_b, error;

»  a=0.707;//! Random values
»  b=0.1234;

»  cos_a_plus_b=cos(a)*cos(b)-sin(a)*sin(b);

»  error=cos_a_plus_b-cos(a+b);

»  printf("I calculate cos(%g) to be %g\n", a+b, cos_a_plus_b);
»  printf("with an error of %g\n", error);

»  return 0;
}

Data