题目

Two disabled Chinese citizens who have planted trees   61   (voluntary) in Northern China have been voted (投票) by Chinese netizens as their inspiring heroes. The vote   62   (organize) by Xinhua News Agency since 2011 to inspire moral strength in the public by highlighting the good deeds of ordinary people. The event is the first of its kind   63   (find) heroes among grassroots people. Jia Wenqi and Jia Haixia, both of   64   are aged over 50, have arm and sight disabilities. Disabled   65   they are, they have planted over 10,000 trees in Jingxing County over （2010•哈尔滨）体育课上，老师用绳子围成一个周长为30米的游戏场地，围成的场地是如图所示的矩形ABCD．设边AB的长为x（单位：米），矩形ABCD的面积为S（单位：平方米）．（1）求S与x之间的函数关系式（不要求写出自变量x的取值范围）；（2）若矩形ABCD的面积为50平方米，且AB＜AD，请求出此时AB的长．