diff --git a/evaldo/builtins_math.go b/evaldo/builtins_math.go
index e72ca2a8..760d8371 100644
--- a/evaldo/builtins_math.go
+++ b/evaldo/builtins_math.go
@@ -183,6 +183,46 @@ var Builtins_math = map[string]*env.Builtin{
 			}
 		},
 	},
+	"acos": {
+		Argsn: 1,
+		Doc:   "Returns the arccosine.",
+		Fn: func(ps *env.ProgramState, arg0 env.Object, arg1 env.Object, arg2 env.Object, arg3 env.Object, arg4 env.Object) env.Object {
+			switch val := arg0.(type) {
+			case env.Integer:
+				if val.Value < -1.0 || val.Value > 1.0 {
+					return MakeBuiltinError(ps, "Invalid input: Acos is only defined for -1 <= x <= 1.", "acos")
+				}
+				return *env.NewDecimal(math.Acos(float64(val.Value)))
+			case env.Decimal:
+				if val.Value < -1.0 || val.Value > 1.0 {
+					return MakeBuiltinError(ps, "Invalid input: Acos is only defined for -1 <= x <= 1.", "acos")
+				}
+				return *env.NewDecimal(math.Acos(val.Value))
+			default:
+				return MakeArgError(ps, 2, []env.Type{env.IntegerType, env.DecimalType}, "acos")
+			}
+		},
+	},
+	"acosh": {
+		Argsn: 1,
+		Doc:   "Returns the inverse hyperbolic cosine.",
+		Fn: func(ps *env.ProgramState, arg0 env.Object, arg1 env.Object, arg2 env.Object, arg3 env.Object, arg4 env.Object) env.Object {
+			switch val := arg0.(type) {
+			case env.Integer:
+				if val.Value < 1.0 {
+					return MakeBuiltinError(ps, " Acosh is only defined for x >= 1.", "acosh")
+				}
+				return *env.NewDecimal(math.Log(float64(val.Value) + math.Sqrt(float64(val.Value)*float64(val.Value)-1)))
+			case env.Decimal:
+				if val.Value < 1.0 {
+					return MakeBuiltinError(ps, " Acosh is only defined for x >= 1.", "acosh")
+				}
+				return *env.NewDecimal(math.Log(val.Value + math.Sqrt(val.Value*val.Value-1)))
+			default:
+				return MakeArgError(ps, 2, []env.Type{env.IntegerType, env.DecimalType}, "acosh")
+			}
+		},
+	},
 	"pi": {
 		Argsn: 0,
 		Doc:   "Return Pi constant.",
diff --git a/tests/basics.rye b/tests/basics.rye
index 36261941..3bcd9733 100644
--- a/tests/basics.rye
+++ b/tests/basics.rye
@@ -22,14 +22,16 @@ section "Printing functions"
 	{
 		stdout { prns 123 } "123 "
 	}
-	group "print\val"
-	mold\nowrap ?print\val
-	{ { object } }
-	{
-		stdout { print\val 33 "value is: {{}}" } "value is: 33" + newline
+	
+	;group "print\val"
+	;mold\nowrap ?print\val
+	;{ { object } }
+	;{
+	;	stdout { print\val 33 "value is: {{}}" } "value is: 33" + newline
 		; stdout { print\val "OK" "value is: {{}}" } "value is: 33" + newline ; TODO-BUG quotes the string
 		; stdout { { "Jane Austen" } print\val "Hello {{}}!" } "value is: 33" + newline
-	}
+	;}
+	
 	; group "print-ssv"
 	; mold\nowrap ?print-ssv
 	; { { function } }
diff --git a/tests/misc.rye b/tests/misc.rye
index c421d19d..c2c8d477 100644
--- a/tests/misc.rye
+++ b/tests/misc.rye
@@ -338,6 +338,23 @@ section "Math functions"
 		equal { do\in math { abs 99.0 } } 99.0
 	}
 
+	group "acos"
+	mold\nowrap "" 
+	{ { string } }
+	{
+		equal { do\in math { acos -1 } } 3.141592653589793
+		equal { do\in math { acos 0.5 } } 1.0471975511965976
+		equal { do\in math { acos 1 } } 0.000000
+	}
+
+	group "acosh"
+	mold\nowrap "" 
+	{ { string } }
+	{
+		equal { do\in math { acosh 1 } } 0.000000
+		equal { do\in math { acosh 2 } } 1.3169578969248166
+	}
+
 	; TODO add sin and cos ... need PI constant
 	
 }