The Belt Pyramid Cairo ____ ancient Egypt’s first attempt to build a smooth-sided pyramid. A. has been believed to have been B. was believed to be C. is believed to have been D. is believed to be
【简答题】如图:已知椭圆A,B,C是长轴长为4的椭圆上三点,点A是长轴的一个端点,BC过椭圆的中心O,且 AC ? BC =0,| BC |=2| AC | . (Ⅰ)求椭圆的标准方程; (Ⅱ)如果椭圆上两点P,Q使得直线CP,CQ与x轴围成底边在x轴上的等腰三角形,是否总存在实数λ使 PQ =λ AB ?请给出证明.