http://go-tour-jp.appspot.com/#24

answer_exercise_gotour_24.go

package main

import (
    "fmt"
    "math"
)

func Sqrt(x float64) float64 {
    z := 1.0
    for {
        zz := z - ((z*z - x)/(2*x))
        if math.Abs(z - zz) < 0.0000001 {
            return z
        }
        z = zz
    }
    return z
}

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