Added cosine function for trigonometric calculations

src/main/c/main_taschenrechner.c

@@ -29,6 +29,12 @@ double sineFunction(double angle) {
 }
 
 
+double cosineFunction(double angle) {
+    // Convert degrees to radians for trigonometric functions
+    return cos(angle * M_PI / 180.0);
+}
+
+
 // scientificmode
 
 int scientificMode(){

src/main/c/taschenrechner.h

@@ -11,6 +11,8 @@ int divide(int a, int b);
 
 double sineFunction(double angle);
 
+double cosineFunction(double angle);
+
 int mode(int userChoice);
 
 int displayMenu();