3.21×6.54所得的积是____位小数．

已知：过⊙O外的定点P作⊙O的两条切线，分别切⊙O于A、B，在劣弧$\stackrel{\wedge }{AB}$上任取一点C，经过点C作⊙O的切线，分别交PA，PB于点D、E．

求证：（1）△PDE的周长是定值（PA+PB）；

（2）∠DOE的大小是定值（$\frac{1}{2}$∠AOB）．

阅读理解

Here is some information about some of the wonders of the world. I am sure you will be interested in them.

The Pyramids of Egypt

There are about eighty pyramids known today. The most well-known was for Pharaoh(法老) Khufu. It is known as the “Great Pyramid”. It’s also the largest one. Workers used about 2. 3 million blocks of stone to build it. (A)人们用了20年的时间才建成。

The Hanging Garden of Babylon

The garden sat on a hill. It was a large and wonderful structure(建筑物). Many parts of the garden were high up on large columns(支柱). There were many big and green trees with lovely flowers. Although there are many different ideas about the garden, we are not sure whether this wonder ever existed(存在)!

The Lighthouse of Alexandria

The Lighthouse of Alexandria was designed about    2, 000 years ago. It was in Egypt, too. It was one of the ancient wonders of the world, about 135 metres high. It was once the highest building in the world. Although it doesn’t exist now, many people come to see its relic(遗址) every year.