由下列不等式：  $1>\frac{1}{2}$， $1+\frac{1}{2}+\frac{1}{3}>1$，  $1+\frac{1}{2}+\frac{1}{3}+\cdots \cdots +\frac{1}{7}>\frac{3}{2}$，  $1+\frac{1}{2}+\frac{1}{3}+\cdots \cdots +\frac{1}{15}>2$， $\cdots \cdots$，你能得到一个怎样的一般不等式？并加以证明．

若平行四边形的周长为28㎝，两邻边之比为4：3，则其中较长的边长为（   ）

    In many western schools, sports day is a big event. Children take part in competitive (竞争性的)sports and try to break school records and take the first place.

    Sports days, or sports meetings, are usually held in the warmer seasons, either at the beginning or the end of the school year. They are also called field days. Primary school sports days are fun. They usually have activities such as the egg and the spoon race and the sack race. Other events include the skipping race (跳绳) and the three-legged race. In middle and high schools, sports days include many of the common track and field events (田径项目).  They are more serious and competitive than primary school ones.

    Students' parents and other relatives also come to the school on sports days. They watch children play. Many schools in the West have "mothers and fathers" races for parents to take part in!

    Although sports days are exciting, they also have some problems. According to some reports in the USA, sports days have become too competitive to be good for students. Some parents put too much pressure on the children. Some schools don't have "mothers and fathers" races any more as there is much fighting and cheating.

根据短文内容回答下列问题。

中共十一届三中全会是共和国历史上的伟大转折，这里的“转折”最主要是指（    ）