题目

All the training you have been doing seem to have____ handsomely.      A. put off                   B. called off            C. paid off                         D. broken off 答案：C7．已知数列{an}满足：a1=1，an=$\frac{1}{3}{a_{n-1}}+\frac{2}{{{3^{n-1}}}}-\frac{2}{3}（{n≥2}）$，设bn=3n-1（an+1）．（Ⅰ）证明：{bn}是等差数列；（Ⅱ）求数列{an}的前n项和．