题目

Trees are one of the oldest citizens (公民) of our Earth. They keep our air clean, reduce noise pollution, improve water quality and provide food and building materials. Just like us, trees change a lot as they grow. At l to 3 years old, young trees learn how to protect themselves. For example, many trees grow thorns (刺) to warn animals not to go near. Most young trees have large, deep green leaves so that they can catch enough sunlight and change it into their food and energy. When trees are 4 years old, they begin to grow very quickly and become strong enough to face challenges(挑战). 如图，在△ABC中，点D、E分别在BC、AC上，BE平分∠ABC，DE∥BA，若AB=7，BC=8．则线段DE的长度为________．