题目

 _______ of the task _______ been completed by the end of last month.   A. Two-thirds; has     B. Two-third; had          C. Two thirds; had        D. Two-third; has 答案：C在数列中，a1＝2，an＋1＝4an－3n＋1，n∈N*.(1)证明数列是等比数列；(2)求数列的前n项和Sn；(3)证明不等式Sn＋1≤4Sn，对任意n∈N*皆成立