Newton's method, solving https://tour.golang.org/flowcontrol/8

newtons.go

package main

import (
	"fmt"
	"math"
)

func Sqrt(x float64) float64 {
	z := 1.0
	l := 0.0
	for math.Abs(z-l)>9e-15 {
		l = z
		z = z - (z*z - x)/(2*z)
	}
	return z
}

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