diff --git a/src/trig_functions/trig_functions.h b/src/trig_functions/trig_functions.h
index 8b13789..e15575b 100644
--- a/src/trig_functions/trig_functions.h
+++ b/src/trig_functions/trig_functions.h
@@ -1 +1,9 @@
+#ifndef trig_functions_H
+#define trig_functions_H
+
+double calculate_sin(double angle);
+double calculate_cos(double angle);
+double calculate_tan(double angle);
+
+#endif