题目

若一个椭圆长轴的长度、短轴的长度和焦距成等差数列,则该椭圆的离心率是( ) (A)   (B)   (C)   (D) 答案：B 解析:由题意可知,2a,2b,2c成等差数列, ∴4b=2a+2c,即a+c=2b, 又a2-c2=b2,∴=a2-c2, 即5c2+2ac-3a2=0, ∴5e2+2e-3=0, 解得e=或e=-1(舍去).30. – Why does the boss look worded?– Tom left the company a month ago and his work unfinished ever since.A．has been leftB．was leftC．leftD．is leaving