噢系壹月拾贰日在米国考的，噢遇到滴输血题，请整理者收纳

cherry12345

Thanks so much SDCAR.  Is the first question chose D?

sdcar2010

第一题 DS:  F(x^2) 是不是小于等于 [F(x)]^2
1)    f(x)= - x
2)    X大于等于0 的时候  f（x）= x       当x 小于零的时候，f（x）= - x

1) f(x^2) = -x^2 <=0; [f(x)]^2 = [-x]^2 >=0.  Sufficient.
2) x>=0: f(x^2) = x^2; [f(x)]^2 = [x]^2. So f(x^2)=[f(x)]^2.
   x<0: same as condition 1. f(x^2)<=[f(x)]^2.
   Sufficient.

Choose D.