题目

Last month, Justin Valdez, a college student, was shot in a subway station in San Francisco. The surveillance video(监控录像) showed that before Justin was killed, the killer pulled out his gun several times, and even wiped his nose with it. However, nobody noticed the killer. The surrounding passengers all focused on their cell phones. We may see the same scene everywhere in our life: in subway stations, restaurants, elevators and so on, people are watching their cell phones. From a family reunion to a date with a friend, people couldn’t stop checking their twitter and faceboo某电影院有20排座位，已知第一排有18个座位，后面一排比前一排多2个座位，写出第n排座位数与排数n之间的关系式为______．