答案：a_1+a_2+a_3+a_4=0

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单项选择题
3. ________ offline CIIE, online CIIE is popular, such as cloud registration, cloud signing, cloud booth, cloud forum and other internet forms in the intelligent CIIE (智慧进博).
A、Beside
B、Besides
C、Except
D、Except for
判断题
CTCS-1级列控系统采用了阶梯速度控制方式。（）
答案：错误