There was a bridge crossing a big river from west to east. It1a man five minutes 2 it. There was a pavilion(凉亭)3the middle of the bridge with a soldier4. He came out5three minutes.6he saw somebody crossing the bridge, he would tell him to go7, not permitting(限制)him to cross. A clever man thought of a8idea, and crossed the9at last. What10did the clever man think of to cross the bridge?

在复平面内，复数3﹣4i，i（2+i）对应的点分别为A、B，则线段AB的中点C对应的复数为（　　）

（Ⅰ）试推导{a_n}的前n项和公式；

（Ⅱ） 设q≠1，证明数列{a_n+1}不是等比数列．