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diff --git a/src/math/sinh.go b/src/math/sinh.go
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+++ b/src/math/sinh.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 sine and cosine.
+
+	The exponential func is called for arguments
+	greater in magnitude than 0.5.
+
+	A series is used for arguments smaller in magnitude than 0.5.
+
+	Cosh(x) is computed from the exponential func for
+	all arguments.
+*/
+
+// Sinh returns the hyperbolic sine of x.
+//
+// Special cases are:
+//	Sinh(±0) = ±0
+//	Sinh(±Inf) = ±Inf
+//	Sinh(NaN) = NaN
+func Sinh(x float64) float64
+
+func sinh(x float64) float64 {
+	// The coefficients are #2029 from Hart & Cheney. (20.36D)
+	const (
+		P0 = -0.6307673640497716991184787251e+6
+		P1 = -0.8991272022039509355398013511e+5
+		P2 = -0.2894211355989563807284660366e+4
+		P3 = -0.2630563213397497062819489e+2
+		Q0 = -0.6307673640497716991212077277e+6
+		Q1 = 0.1521517378790019070696485176e+5
+		Q2 = -0.173678953558233699533450911e+3
+	)
+
+	sign := false
+	if x < 0 {
+		x = -x
+		sign = true
+	}
+
+	var temp float64
+	switch {
+	case x > 21:
+		temp = Exp(x) * 0.5
+
+	case x > 0.5:
+		ex := Exp(x)
+		temp = (ex - 1/ex) * 0.5
+
+	default:
+		sq := x * x
+		temp = (((P3*sq+P2)*sq+P1)*sq + P0) * x
+		temp = temp / (((sq+Q2)*sq+Q1)*sq + Q0)
+	}
+
+	if sign {
+		temp = -temp
+	}
+	return temp
+}
+
+// Cosh returns the hyperbolic cosine of x.
+//
+// Special cases are:
+//	Cosh(±0) = 1
+//	Cosh(±Inf) = +Inf
+//	Cosh(NaN) = NaN
+func Cosh(x float64) float64
+
+func cosh(x float64) float64 {
+	x = Abs(x)
+	if x > 21 {
+		return Exp(x) * 0.5
+	}
+	ex := Exp(x)
+	return (ex + 1/ex) * 0.5
+}