⑴How old are you ?____                     A. He is a doctor.

⑵What's your name?____                   B. I have a cold.

⑶What does he do?____                    C. My name is Anna.

⑷What are they doing?____               D. I'm ten years old.

⑸What's wrong with you ?____          E. They are waiting outside the teacher's office.

⑹What does she see?____                  F. She sees some birds.

一组按规律排列的式子：$\frac{2}{a}$，$-\frac{5}{a^2}$，$\frac{10}{a^3}$，$-\frac{17}{a^4}$，$\frac{26}{a^5}$，…，其中第7个式子是____ ，第n个式子是____ （用含的n式子表示，n为正整数）．