http://tour.golang.org/#25

25.go

package main

import (
    "fmt"
    "math"
)

func Sqrt(x float64) (z float64) {
    z = 1.0
    for i := 0; i < 100000000; i++ {
        prev:= z
    	z = z - ((z * z - x) / 2 * z)
        if diff := math.Abs(prev - z); diff < 0.0001 {    
            fmt.Println("good enough!", i, diff)
            break;
        }
    }
    return
}

func main() {
    fmt.Println("per Newton:", Sqrt(2))
    fmt.Println("per math lib:", math.Sqrt(2))
}