当子系统只能处于正常工作和不工作两种状态时，我们可以采用下图所示的并联模型，若单个子系统的可靠性都为0.8时，三个子系统并联后的系统可靠性为______。

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A Web browser is simply a terminal emulator, designed to display text on a screen. The two essential differences between an ordinary terminal emulator and a Web browser are that the browser knows how to deal with(71), and that it has a mechanism for(72)graphical files. Display text, display graphics, and(73)hyperlinks-there's 99 percent of the(74)value. That's not to say that the manufacturers didn't go all-out to attach a hyperactive efflorescence of useless capabilities onto their browsers. Remember when media channels in the browser were a big deal, instead of the clutter you can't wait to delete from your favorites of bookmarks menu? Remember when client-side Java applets were supposed to become the preferred(75)for application development? Remember frames and all their nasty side effects?

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假设某程序语言的文法如下：

 S→a|b|(T)

 T→TdS|S

 其中，V_T={a，b，d，(，))；V_N={S，T}，S是开始符号。考察该文法，句型(Sd(T)db)是S的一个(28)。

 其中(29)是最左素短语，(30)是该句型的直接短语。

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