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diff --git a/libgo/go/math/tanh.go b/libgo/go/math/tanh.go
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+++ b/libgo/go/math/tanh.go
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+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package math
+
+
+/*
+	Floating-point hyperbolic tangent.
+
+	Sinh and Cosh are called except for large arguments, which
+	would cause overflow improperly.
+*/
+
+// Tanh computes the hyperbolic tangent of x.
+func Tanh(x float64) float64 {
+	if x < 0 {
+		x = -x
+		if x > 21 {
+			return -1
+		}
+		return -Sinh(x) / Cosh(x)
+	}
+	if x > 21 {
+		return 1
+	}
+	return Sinh(x) / Cosh(x)
+}