平方根的近似求法，编写sqrt方法，数num的平方根可通过反复计算下列公式求得nextguess=（lastguess+(num/lastguess)）/2-Approximate approach to the square root, sqrt method of preparation, the square root of the number of num can be obtained by repeatedly calculating the following formula nextguess = (lastguess+ (num/lastguess))/2