통계학 입문 https://enook.jbnu.ac.kr/contents/148/#!/p/34
평균 변화율 https://ko.khanacademy.org/math/algebra1/x2f8bb11595b61c86:functions/x2f8bb11595b61c86:average-rate-of-change-word-problems/a/average-rate-of-change-review
