题目

 (衡阳市八中2008届高三模拟试题B篇) Skipping classes, particularly big lectures where an absence is likely to go undetected, is a tradition among college undergraduates. These days, however, some professors say they’re seeing more absenteeism, as students make the most of new technologies as learning aids. Americ Azevedo taught an “ Introduction to Computers ” at the University of California, Berkeley, US last semester. By visiting the course’s website, the 200 enrolled students could download audio recordings or watch digital videos of the lectures, as well as read已知等差数列{an}中，a1=1，前10项和S10=100；（1）求数列{an}的通项公式； （2）设log2bn=an，证明{bn}为等比数列，并求{bn}的前四项之和．（3）设cn=bn+an，求{cn}的前五项之和．