题目

若一个等差数列{an}前3项的和为34,最后3项的和为146,且所有项的和为390,则这个数列共有__________项. 答案：思路解析:设该数列共有n项,则有a1+a2+a3=34,an+an-1+an-2=146,两式相加得(a1+an)+(a2+an-1)+(a3+an-2)=180,又a1+an=a2+an-1=a3+an-2,故a1+an=60.又Sn=,所以=390,n=13.答案:13 Gina loves singers _________ write their own music． [　　] A． which B． who C． has worked