题目

    Australia is the smallest continent in the world. It is a little smaller than China. It is on the Southern Hemisphere of the earth. Australia is big, but its population is small. The population of Australia is nearly as large as that of Shanghai.     Enough laws have been made to fight against pollution. The cities in Australia have little air or water pollution. The sky is blue and the water is clean. You can clearly see fish in the rivers. Plants grow very well.     Last month we visited Perth, the biggest city in Western Australia, and went to a wild flower exhibition. Ther如图，已知⊙O是△ABC的外接圆，CD是AB边上的高，AE是⊙O的直径．求证：AC•BC=AE•CD．