题目

The world is filled with smart, talented, educated and gifted people. We meet them every day. A few days ago, my car was not running well. I pulled it into a garage and the young mechanic had it fixed in just a few minutes. He knew what was wrong by simply listening to the engine. I was amazed. The sad truth is, great talent is not enough.      I am constantly shocked at how little talented people earn. I heard the other day that less than 5 percent of Americans earn more than $100, 000 a year. A business consultant who specializes in the medical trade was telling me how many do10．如图，在△ABC中，AB=AC=10，BC=12，矩形DEFG的顶点位于△ABC的边上，设EF=x，SDEFG=y．（1）请你用含x的式子表示线段DE；（2）写出y与x的函数关系式，并注明自变量x的取值范围；（3）当x取何值时，y的值最大？最大值是多少？