A:甲烷和乙烯
B:苯和四氯化碳
C:水和苯
D:氯化钠和硝酸钾
公差d≠0的等差数列{an}的前n项和为Sn,若向量OA1→=ai,Siii=1,2,3,t为实数,若A1A2→=tA2A3→,则t=( )
Write at least 60 words about the topic ’My Opinion on Having Lessons at Weekends’.(以“双休日上课之我见一为题写一篇不少于60个词的短文,标点符号不占格。)As a Grade Nine student, you have lessons at weekends. Do you think it is a good idea to improve your study? Why or why not? Give at least two reasons to support your opinion.作为一名九年级的学生,你双休日还要去学校上课。你觉得这是提高学习成绩的好办法吗? 请你陈述支持你观点的理由。(至少两条)(注意:短文中不得出现考生的姓名、校名及其它相关信息,否则不予评分。)
最大的一位数乘最小的三位数,积是____。