说句实在话，关于这个问题，局外人不知道其中的苦衷，…直到2014年8月,Keisler微积分教材第三版出现，允许自由段落拷贝，拷贝的段落呈现黑底白字，有助于读者长期阅读，不伤害视力，于今年6月10日，袁萌决定微积分互联网大课堂正式开课！大课堂的讲课黑板是黑底白字！与校园小课堂完全一样。
例如，该教材的第一章第二节，可以抓取下来阅读，如下：（以便保护视力）
1.2 The openinterval (a, b)
For both open and closed intervals, thenumber a is called the lower endpoint, and b the upper endpoint. The differencebetween the closed interval [a, b] and the open interval (a, b) is that theendpoints a and b are elements of [a, b] but are not elements of (a, b). When a:::;; x :::;; b, we say that x is between a and b; when a < x < b, we saythat x is strictly between a and b. Three other types of sets are also countedas open intervals: the set (a, YJ) of all real numbers x greater than a; theset (-OJ, b) of all real numbers x less than b, and the whole real line R. Thereal line R is sometimes denoted by (- x, 'lJ ). The symbols JJ and - :x::,read "infinity" and "minus infinity," do not stand fornumbers; they are only used to indicate an interval with no upper endpoint, orno lower endpoint. Besides the open and closed intervals, there is one otherkind of interval, called a half open interval. The set of all real numbers xsuch that a :::;; x < b is a half open interval denoted by [a, b). The setof all real numbers x such that a :::;; x is also a half-open interval and iswritten [a, ). Here is a table showing the various kinds ofintervals.
感言：00后大学生都是国家下一代栋梁之才，长大了，视力不好，对不住我们的子孙。
例如，该教材的第一章第二节，可以抓取下来阅读，如下：（以便保护视力）
1.2 The openinterval (a, b)
For both open and closed intervals, thenumber a is called the lower endpoint, and b the upper endpoint. The differencebetween the closed interval [a, b] and the open interval (a, b) is that theendpoints a and b are elements of [a, b] but are not elements of (a, b). When a:::;; x :::;; b, we say that x is between a and b; when a < x < b, we saythat x is strictly between a and b. Three other types of sets are also countedas open intervals: the set (a, YJ) of all real numbers x greater than a; theset (-OJ, b) of all real numbers x less than b, and the whole real line R. Thereal line R is sometimes denoted by (- x, 'lJ ). The symbols JJ and - :x::,read "infinity" and "minus infinity," do not stand fornumbers; they are only used to indicate an interval with no upper endpoint, orno lower endpoint. Besides the open and closed intervals, there is one otherkind of interval, called a half open interval. The set of all real numbers xsuch that a :::;; x < b is a half open interval denoted by [a, b). The setof all real numbers x such that a :::;; x is also a half-open interval and iswritten [a, ). Here is a table showing the various kinds ofintervals.
感言：00后大学生都是国家下一代栋梁之才，长大了，视力不好，对不住我们的子孙。
袁萌 6月21日