题目

Human life has not only been changed within the past few hundred years．it has been revolutionized．In the old days there was little difference between the houses．dress and food of those who were chiefs and those who were their servants．There was, to a large degree．social equality．The contrast（差别）today between the palaces of the millionaires and the cottages of the laborers shows us the change which has come with civilization（文明）．       This change．however．is not to be regretted．but welcome as highly beneficial（helpful）．For progress there must be h等差数列{an}的前n项和记为Sn．已知a10=30，a20=50．（Ⅰ）求通项an；（Ⅱ）若Sn=242，求n．