说明：  用For/Next循环语句编写程序计算自然对数之和，并要求先定义求a到b的自然对数之和的函数为Nlogarithm(a,b),然后单击“求自然对数之和”按钮时调用该函数。
(With a For/Next loop programming to calculate the sum of the natural logarithm and requires a to b seeking to define the natural logarithm of the sum function Nlogarithm (a, b), and then click "seeking natural logarithm of the sum" button when the function is called. )