已知平方根迭代公式Xn+1=(Xn+a/Xn)/2，设X=a/2。编写程序输入a值计算其平方根。迭代的结束条件是x n+1-xn<10-5-The square root known iteration formulas Xn+ 1 = (Xn+ a/Xn)/2, set X = a/2. Write a program input a value are calculated the square root. The end of the iteration condition is x n+ 1-xn < 10-5