题干
根据句子意思，用括号中所给词的正确形式填空，每空填一词。
Seven ____ (twelve) of the students have taken part in the tree-planting activity since Monday.
答案(点此获取答案解析）
twelfths
根据句子意思，用括号中所给词的正确形式填空，每空填一词。
Seven ____ (twelve) of the students have taken part in the tree-planting activity since Monday.
是等差数列
的前
项和，且
，
，则
等于（ ）