题目

There are more and more graduates in our society, but do you believethat a diploma is notguaranteeof efficiency necessarily? 答案：【1】a 如图， Rt△ABC中，∠ABC=90°，以AB为直径的⊙O交AC于点D，E为BC边的中点，连接DE. （1）求证：DE与⊙O 相切. （2）若tanC=，DE=2，求AD的长．