ついでに 3乗根の Math.cbrt も作ったんですがどうでしょうか。

cbrt は C99 で定義されています。

Index: math.c
===================================================================
--- math.c	(revision 15410)
+++ math.c	(working copy)
@@ -402,6 +402,20 @@ math_sqrt(VALUE obj, VALUE x)
 
 /*
  *  call-seq:
+ *     Math.cbrt(numeric)    => float
+ *  
+ *  Returns the cube root of <i>numeric</i>.
+ */
+
+static VALUE
+math_cbrt(VALUE obj, VALUE x)
+{
+    Need_Float(x);
+    return DOUBLE2NUM(cbrt(RFLOAT_VALUE(x)));
+}
+
+/*
+ *  call-seq:
  *     Math.frexp(numeric)    => [ fraction, exponent ]
  *  
  *  Returns a two-element array containing the normalized fraction (a
@@ -603,6 +617,7 @@ Init_Math(void)
     rb_define_module_function(rb_mMath, "log2", math_log2, 1);
     rb_define_module_function(rb_mMath, "log10", math_log10, 1);
     rb_define_module_function(rb_mMath, "sqrt", math_sqrt, 1);
+    rb_define_module_function(rb_mMath, "cbrt", math_cbrt, 1);
 
     rb_define_module_function(rb_mMath, "frexp", math_frexp, 1);
     rb_define_module_function(rb_mMath, "ldexp", math_ldexp, 2);
Index: configure.in
===================================================================
--- configure.in	(revision 15410)
+++ configure.in	(working copy)
@@ -649,7 +649,7 @@ esac
 AC_FUNC_MEMCMP
 AC_REPLACE_FUNCS(dup2 memmove strerror strftime\
 		 strchr strstr crypt flock vsnprintf\
-		 isnan finite isinf hypot acosh erf tgamma lgamma_r \
+		 isnan finite isinf hypot acosh erf tgamma lgamma_r cbrt \
                  strlcpy strlcat)
 AC_CHECK_FUNCS(fmod killpg wait4 waitpid fork spawnv syscall chroot fsync getcwd eaccess\
 	      truncate chsize times utimes utimensat fcntl lockf lstat\
Index: missing/cbrt.c
===================================================================
--- missing/cbrt.c	(revision 0)
+++ missing/cbrt.c	(revision 0)
@@ -0,0 +1,10 @@
+#include <math.h>
+
+double cbrt(double x)
+{
+    if (x < 0)
+        return -pow(-x, 1/3.0);
+    else
+        return pow(x, 1/3.0);
+}
+
-- 
[田中 哲][たなか あきら][Tanaka Akira]