added cmplx cube example

exercises/04-cube/go.go

@@ -0,0 +1,23 @@
+package main
+
+import (
+	"fmt"
+	"math/cmplx"
+)
+
+const threshold = 1e-5
+
+func Cbrt(x complex128) (z complex128) {
+	z = x
+	for delta, last := 1e9, z; delta > threshold; last = z {
+		z -= (z*z*z - x) / (3 * z * z)
+		delta = cmplx.Abs(z - last)
+	}
+	return
+}
+
+func main() {
+	n := 12392.13 + 1982i
+	fmt.Println("calculated ", Cbrt(n))
+	fmt.Println("expected   ", cmplx.Pow(n, 1/3.0))
+}