Add `tanh' with documentation.

svn: r14250

collects/scheme/math.ss

@@ -7,7 +7,7 @@
 (provide pi
          sqr
          sgn conjugate
-         sinh cosh)
+         sinh cosh tanh)
 
 (define (sqr z) (* z z))
 
@@ -29,3 +29,5 @@
 
 (define (cosh x)
   (/ (+ (exp x) (exp (- x))) 2.0))
+
+(define (tanh x) (/ (sinh x) (cosh x)))

collects/scribblings/reference/numbers.scrbl

@@ -890,6 +890,10 @@ Returns the hyperbolic sine of @scheme[z].}
 
 Returns the hyperbolic cosine of @scheme[z].}
 
+@defproc[(tanh [z number?]) number?]{
+
+Returns the hyperbolic tangent of @scheme[z].}
+
 @; ----------------------------------------------------------------------
 
 @close-eval[math-eval]