My incomplete answer for http://go-tour-jp.appspot.com/#47

gistfile1.go

// ! incomplete
package main

import (
    "fmt"
    "math/cmplx"
)

func Cbrt(x complex128) complex128 {
    var z complex128 = 1
    for i := 0; i < 10; i++ {
        z = z - (cmplx.Pow(z, 3) - x)/(3 * cmplx.Pow(z, 2))
    }
    return z
}

func main() {
    fmt.Println(Cbrt(2))
    fmt.Println(cmplx.Pow(2, 1/3))
}