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Computers are very important for the students now. For the city students, it's very easy to find a place to surf the Internet(网上冲浪). And for the village students, they can go to the Internet bar(网吧). Most of them spend more than two hours there, and some of them even spend more than eight hours there every day. They can do their homework, download(下载) some information for their homework, download music, send emails and play games.Most of them spend too much time playing games, so it's bad for their study.

若X是离散型随机变量，$P\left(X=x_1\right)=\frac{2}{3},P\left(X=x_2\right)=\frac{1}{3}$，且$x_1<x_2$，又已知$E\left(X\right)=\frac{4}{3},D\left(X\right)=\frac{2}{9}$，则$x_1+x_2$的值为（   ）