78. DS:一个含有10个数字(1,3,5,7,9,11,13,15,17,19)的数列的标准方差问题。问方差是否不变?
a: 抽掉2个数后,mean不变;
b: 抽掉2个数后,sum为90。
我选D
[讨论] D
REMARK:我觉得B不对
87. x^2+6x+q, what is the value of q?
1). X^2+6x+q=x^2+rx+r-2. some r is satisfied for this quotation. All variable X is satisfied for this
2). x^2+6x=Q=(x+m)(x+n), some m and n is satisfied for this quotation. All variable X is satisfied for this
My Ans.: E
[讨论]E
REMARK:1)为什么不对?