题干
选择合适的词组, 并用其正确的形式填在横线上。
run across the road put out be born go on a picnic leave for
be made of have to take one’s advice more than plenty of
选择合适的词组, 并用其正确的形式填在横线上。
run across the road put out be born go on a picnic leave for
be made of have to take one’s advice more than plenty of
为等差数列，
是其前n项和，且
，则
的值为（ ）
