Approximating the square root

Implement the sqrt function. The square root of a number, num, can be approximated by repeatedly performing a calculation using the following formula :

nextGuess = (lastGuess + (num / numGuess)) /2

The initial guess can be any positive value (e.g., 1). This value will be the starting vale for lastGuess. If the difference between nextGuess and lastGuess is less than a very small number, such as 0.0001, you can claim that nextGuess is the approximated square root of num. If not, nextGuess becomes the lastGuess and the approximation process continues.