diff --git a/Angle.cpp b/Angle.cpp
index 166a88e..676a7e8 100644
--- a/Angle.cpp
+++ b/Angle.cpp
@@ -212,6 +212,11 @@ float AngleOf<T>::Tan(AngleOf<T> a) {
   return tanf(a.InRadians());
 }
 
+template <typename T>
+AngleOf<T> Passer::LinearAlgebra::AngleOf<T>::Acos(float f) {
+  return AngleOf<T>::Radians(acosf(f));
+}
+
 template <>
 AngleOf<float> AngleOf<float>::CosineRuleSide(float a,
                                               float b,
diff --git a/Angle.h b/Angle.h
index b65633b..c4561e5 100644
--- a/Angle.h
+++ b/Angle.h
@@ -59,9 +59,11 @@ class AngleOf {
                                 AngleOf<T> toAngle,
                                 AngleOf<T> maxAngle);
 
-  static float Cos(AngleOf<T>);
-  static float Sin(AngleOf<T>);
-  static float Tan(AngleOf<T>);
+  static float Cos(AngleOf<T> a);
+  static float Sin(AngleOf<T> a);
+  static float Tan(AngleOf<T> a);
+
+  static AngleOf<T> Acos(float f);
 
   static AngleOf<T> CosineRuleSide(float a, float b, AngleOf<T> gamma);
   static AngleOf<T> CosineRuleAngle(float a, float b, float c);