填空题在抛物线y=1-x2与x轴所围成的平面区域内作一内接矩形ABCD，其一边AB在x轴上(如图1-2-4所示)．设AB=2x，矩形面积为S(x)．<br/> <img border="0" alt="" src="https://tuku.weimei168.com/uploads/upimg/20240620/14288429_0.png"><br/> 图1—2—3<br/> <img border="0" alt="" src="https://tuku.weimei168.com/uploads/upimg/20240620/14288429_0_1.png"><br/> 图1—2—4<br/> ①写出S(x)的表达式；<br/> ②求S(x)的最大值．