【简答题】Which of the following statements about sales tax is/are true? 1 Sales tax charged on purchases is an expense to a business if the business is not sales taxregistered.2 Sales tax is charged by all bus...
【简答题】已知椭圆 x 2 a 2 + y 2 b 2 =1(a>b>0) 的离心率 e= 2 2 ,短轴长为2. (1)求椭圆方程; (2)若椭圆与x轴正半轴、y轴正半轴的交点分别为A、B,经过点 (0, 2 ) 且斜率k的直线l与椭圆交于不同的两点P、Q.是否存在常数k,使得向量 OP + OQ 与 AB 共线?如果存在,求k的值;如果不存在,请说明理由.