My answer for http://go-tour-jp.appspot.com/#23

gistfile1.go

package main

import (
    "fmt"
    "math"
)

func Sqrt(x float64) float64 {
    z := 1.0;
    for i := 0; i < 10; i++ {
        z = z - (math.Pow(z, 2) - x) / (2 * z)
    }
    return z
}

func main() {
    fmt.Println(Sqrt(2))
    fmt.Println(math.Sqrt(2))
}