设数列是首项为,公差为的等差数列,其前项和为,且成等差数列。 (Ⅰ)求数列的通项公式; (Ⅱ)记的前项和为,求. 答案:(Ⅰ)∵,,,-------------2分 由成等差数列得,,即, 解得,故;-------------------------------------4分 (Ⅱ), ---------------------------------------5分 法1:, ① ①得,, ② ①②得, , -----------------10分 ∴.------------------------------12分 法2:, 设,记The driver was badly hurt and he needed_________ at once.A. a research B. an operationC. an experiment D. a survey