题目

Have you ever been to the world’s smallest bookstore? The World’s Smallest Bookstore, whose official name is just these three words, sits quietly about 100 miles northeast of Toronto. The bookstore is about 10 feet by 10 feet, so it is easy to imagine how tiny it really is. The bookstore is open 24 hours a day. Inside the bookstore are various books, especially literary books and classic authors’ works. So if you are looking for something less popular, you may get a bit disappointed there. Another special feature of this bookstore is that each book only costs three dollars. All t已知AB为半圆O的直径,且AB=4,C为半圆上一点,过点C作半圆的切线CD,过A点作AD⊥CD于D,交半圆于点E,DE=1.（Ⅰ）证明:AC平分∠BAD;（Ⅱ）求BC的长.