题干
假如这是你的妹妹，请对她进行描述，至少5句话，要求2～3种句型。
答案(点此获取答案解析）
My sister
My sister is a little girl. She is only four years old. She is very lovely. Her hair is brown and short. She wears a red T-shirt and a pair of yellow shorts . We like my sister very much.
My sister is a little girl. She is only four years old. She is very lovely. Her hair is brown and short. She wears a red T-shirt and a pair of yellow shorts . We like my sister very much.
的前n项和为
，若
，
，则
的
