为迎接第十四届中国双黄鸭蛋节,组委会设计了鸭蛋型图徽.图徽外框由半圆和半椭圆组成(如图),半圆的直径为10,椭圆的离心率为,且短轴与半圆的直径重合,图徽内有一矩形区域用于绘画图案,矩形关于椭圆的长轴对称,且顶点在图徽外框上. (Ⅰ)建立适当的直角坐标系,求出半圆的方程和半椭圆的方程; (Ⅱ)根据美学知识,当时达到最佳美观的效果,求达到最佳美观的效果时的长.( ) 12. ―Why is Harvey's mother so happy?―Because only three students , his son Harvey. A. failed the exam;besides B. made progress;except C. made progress;including D. passed the exam;without