算正数a的算术平方根。迭代公式如下xn+1=(xn+a/xn)/2， x 要求前后两次求出的x的差的绝对值小于 10－5 。 a通过键盘输入。 -A count of the positive square root. Iteration formula is as follows xn+1 = (xn+a/xn)/2, the difference between before and after the absolute value of x requires two x is less than 10-5 is obtained. input via a keyboard.