用适当的词补全对话。
A: Do you know we have a new teacher and two new students this semester?
B: Really? What ____ the new teacher look ____?
A: He is an old man. He ____ no hair.
B: Is he ____?
A: Yes. And he ____ glasses.
B: ____ he have a mustache?
A: No, he ____ a beard. He is not good-looking. But he is very kind.
B: What ____ your new classmates? What ____ they ____ like?
A: They are twins. They look the ____. They ____ of medium height. They're not heavy ____ thin. They ____ curly hair.
B: What's the ____ ____ their hair?
A: Well, they ____ blonde hair. They are good-looking.
是等差数列
的前n项和，有
，则
的值为（ ）
