通常情况下，若y是关于x的函数，则y与x的函数关系式可记作y=f（x）．如y=$\frac{1}{2}$x+3记作f（x）=$\frac{1}{2}$x+3，当x=2时，f（2）=$\frac{1}{2}$×2+3=4．下列四个函数中，满足f（a+b）=f（a）+f（b）的函数是（　　）

2010年3月1日施行的《中国保险监督管理委员会行政复议办法》规定，公民、法人或者其他组织认为保监会或其派出机构的具体行政行为侵犯其合法权益的，可以自知道该具体行政行为之日起60日内提出行政复方申请。从公民的角度看，当其合法权益受到侵犯时，应
①自觉维护个人的正当利益                    ②坚持个人利益和国家利益相结合的原则
③依法行使公民的基本政治权利                ④制裁和惩罚侵犯自己权益的行为人

下图是硝酸钾的溶解度曲线。

某同学在20℃时进行了如下实验，得到相应的溶液①~⑤，下列说法正确的是（   ）

阅读理解

       Computers are useful machines．They can help people a lot in their daily life．For example，they can help people to save much time to do much work，and they can help people to work out many problems easily．Our country asks everyone to learn to use computers before the twenty-first century，except the old people．  21·cn·jy·com

      Today more and more families own computers．Parents buy computers for their children．They hope computers can help them improve their studies in school．Yet，many of their children use computers to play games，to watch videos or to chat．Computers can not help children to study but make them fall behind．So computers are locked in boxes by parents．

        In some other countries，even some scientists also hate computers．They say computers let millions of people lose their jobs or bring them a lot of trouble．Will computers really bring trouble to people or can they bring people happiness? It will be decided by today's students themselves!

等差数列 $\left\{a_n\right\}$ 中， $a_6+a_7+a_8=12$ ，则该数列的前 $13$ 项的和 $S_{13}=$ ____.